The Ultimate French Onion Broccoli Cheese Casserole: A Decadent Comfort Food Fusion

If you love the rich, savory depth of French Onion Soup married with the creamy, satisfying textures of a classic broccoli cheese casserole, then you are about to discover your new favorite dish. This French Onion Broccoli Cheese Casserole is the epitome of comfort food sophistication. We take the slow-cooked, deeply caramelized onions that define the classic French soup, fold them into a luxurious, nutmeg-scented Gruyère and white cheddar cheese sauce, and bathe tender broccoli florets in this glorious mixture. Topped with crunchy croutons and extra cheese, this bake delivers an unparalleled depth of flavor. Forget the bland holiday side dish this casserole is ready to be the star of your dinner table.

Making this incredible casserole requires patience, especially during the onion caramelization phase, but trust us, every minute spent stirring is worth the resulting complexity. This article provides a comprehensive guide, from mastering the French onion base to achieving the perfect golden, crunchy topping. Get ready to elevate your weeknight dinner or holiday spread with the best French Onion Broccoli Cheese Casserole you’ve ever tasted.

Why You’ll Absolutely Love This French Onion Broccoli Cheese Casserole (Sophisticated Comfort)

This isn’t just another side dish; it’s a culinary experience. The reason this specific French Onion Broccoli Cheese Casserole recipe garners such rave reviews is its perfect balance between rustic simplicity and gourmet flavor complexity. Here is a breakdown of why this recipe will become a permanent fixture in your kitchen rotation (300-500 words).

The Secret is in the Caramelization

Unlike most speedy casserole recipes that rely on cream of mushroom soup or pre-packaged sauces, the heart of this dish lies in the meticulously prepared caramelized onions. These onions, slowly cooked over nearly an hour, transform from pungent vegetables into sweet, deep, umami-rich golden strands. This process releases natural sugars and creates a fundamental flavor base that no shortcut can replicate. When this luxurious base combines with rich beef broth and a touch of balsamic vinegar, it mimics the famous soup base, providing an intense savory foundation for the entire casserole.

Unbeatable Cheese Blend for Ultimate Flavor

Many broccoli cheese casseroles fall flat using only mild cheddar. We choose a dynamic duo: Gruyère and Sharp White Cheddar. Gruyère is a classic choice for French onion dishes, known for its nutty, earthy notes and superior melting quality. It provides that essential alpine aroma that screams “French Onion.” Sharp white cheddar, meanwhile, adds tang and a rich creaminess, cutting through the sweetness of the onions and ensuring the sauce isn’t too heavy. This combination melts smoothly and sets beautifully when baked, creating a creamy, cohesive filling that hugs the broccoli florets.

Texture, Texture, Texture

A great casserole needs variety. While the interior is velvety and soft, the topping provides the crucial contrast. By using crushed, seasoned croutons (or robust panko breadcrumbs) mixed with extra Gruyère, you achieve a thick, golden crust that delivers a satisfying crunch in every bite. This topping protects the creamy filling while baking and introduces a savory, buttery dimension that complements the soft broccoli and melting cheese.

A Meal or a Show-Stopping Side

Whether you need a vegetarian main course (if omitting the beef broth or substituting with vegetable broth) or an unforgettable side dish for holidays like Thanksgiving or Christmas, this casserole delivers. It’s hearty enough to stand alone and pairs beautifully with roasted chicken, pork tenderloin, or a classic Spanish dish like lasaña de carne con bechamel. The complexity of the flavor profile elevates any meal it graces. The effort put into the foundational flavor base ensures that every spoonful of this French Onion Broccoli Cheese Casserole is a deeply satisfying experience.

Why You Should Try Making This French Onion Broccoli Cheese Casserole Today (The Gourmet Upgrade)

You might already have a go-to broccoli cheese recipe, but we promise this version the French Onion Broccoli Cheese Casserole is the gourmet upgrade you didn’t know you needed. It transforms a humble vegetable side dish into a culinary masterpiece that will impress even the most discerning palates. Here is why you should clear your schedule and start slicing onions now (300-500 words).

Master the Art of Flavor Building

Cooking is about building layers of flavor, and this recipe is a masterclass in that technique. The first layer is the caramelized onions, which provide sweetness and depth. The second layer is the rich deglazing broth, which adds saltiness and umami. The third layer is the velvety béchamel, providing fat and body. Finally, the Gruyère and Sharp Cheddar introduce nuttiness and tang. By preparing these components separately and then combining them, you achieve a depth of flavor that is simply impossible with quick-mix alternatives. Trying this recipe helps hone essential cooking skills, like roux making and deglazing, making you a better home cook.

A Healthy Dose of Nostalgia

Casseroles are inherently nostalgic, reminding us of family dinners and holiday gatherings. This recipe taps into that comfort while simultaneously elevating the quality. It takes the familiar warmth of a classic broccoli bake and infuses it with the sophisticated, earthy notes of French bistro cooking. It’s comforting, yes, but it’s also undeniably elegant. This makes the French Onion Broccoli Cheese Casserole perfect for hosting it’s accessible yet special.

Maximizing Vegetable Appeal

Broccoli is a famously healthy vegetable, but sometimes it needs a little encouragement for enthusiastic consumption. When enveloped in this creamy, savory, onion-rich sauce, even the pickiest eaters will ask for seconds. The intense flavor profile effectively masks any bitterness the broccoli might possess, ensuring every floret is coated in delicious, melting cheese and sweet onion goodness. This is an excellent way to sneak nutritious vegetables into a hearty meal.

Perfect for Make-Ahead Preparation

For those busy weeknights or high-stress holiday cooking days, the French Onion Broccoli Cheese Casserole is a savior. You can prepare the caramelized onions completely ahead of time up to three days and store them in the refrigerator. You can also assemble the entire casserole (minus the crouton topping) 24 hours in advance, cover it, and bake it when ready. This flexibility allows you to focus on other parts of your meal, ensuring a smooth and stress-free cooking experience. If you are preparing for a larger gathering, consider pairing this decadent side dish with a simpler classic, like a Spanish tortilla de patatas, allowing the complexity of the casserole to truly shine.

Ingredients and Essential Utensils for the Best French Onion Broccoli Cheese Casserole

The success of this recipe hinges on high-quality ingredients and the right tools, especially since we are building multiple distinct layers of flavor. Ensuring you have everything prepped before you start (Mise en Place) will make the cooking process much smoother (700-900 words).

#For the Caramelized Onions (French Onion Base)

This is the foundation of the dish, providing the deep, earthy, sweet notes. Do not rush this step!

• 3 large yellow onions, thinly sliced: Yellow onions offer the best balance of sweetness and flavor for caramelizing. Slice them uniformly thin to ensure even cooking.
• 4 tablespoons unsalted butter, divided: Used for initial melting and building the richness of the roux later.
• 1 tablespoon olive oil: Helps prevent the butter from burning during the initial phase of caramelization.
• 1 teaspoon dried thyme: Thyme is crucial; it provides the savory, slightly floral note that defines classic French onion flavor.
• 1 tablespoon balsamic vinegar: This is a secret ingredient! Added at the end, it cuts the richness, adds complexity, and enhances the dark brown color of the caramelized onions.
• 1/2 teaspoon kosher salt and 1/4 teaspoon black pepper: Essential seasoning.
• 1 cup rich beef broth: Use high-quality, low-sodium beef broth. The richness is necessary for the deglazing process and maximizing the umami flavor in the French Onion Broccoli Cheese Casserole.

#For the Casserole Filling

This section focuses on the creamy, cheesy body of the Broccoli Cheese Casserole.

• 6 cups fresh or frozen broccoli florets: If using fresh, make sure they are cut into bite-sized florets. If using frozen, there is no need to thaw they go straight into the mixture.
• 3 tablespoons unsalted butter: Used for creating the roux.
• 3 tablespoons all-purpose flour: This forms the roux (a thickening agent) with the butter, creating the base for the creamy cheese sauce.
• 1 1/2 cups half-and-half (or whole milk): Half-and-half yields a richer, thicker sauce, but whole milk works well for a slightly lighter consistency.
• 1/4 teaspoon nutmeg: A pinch of freshly grated nutmeg is traditional in béchamel-based sauces and adds a warm, subtle complexity that pairs beautifully with Gruyère.
• 1 cup shredded Gruyère cheese: Mandatory for the authentic French onion flavor. Tip: Shred the cheese yourself; pre-shredded varieties contain anti-caking agents that prevent smooth melting.
• 1 cup shredded sharp white cheddar cheese: Adds the necessary tang and classic casserole creaminess.

#For the Topping

The crowning glory of this decadent bake.

• 1 cup crushed seasoned croutons or panko breadcrumbs: Seasoned croutons offer superior crunch and built-in flavor. If using panko, mix them with 1 tablespoon of melted butter and a pinch of salt and pepper before topping.
• 1/2 cup extra shredded Gruyère cheese: Sprinkled over the croutons for optimal melting and browning.

Necessary Utensils and Equipment

Having the right tools ensures efficiency, especially during the long caramelization process and the delicate roux making.

• Large Dutch Oven or Deep Skillet: Essential for the onion caramelization (Step 1). The heavy bottom distributes heat evenly and prevents scorching over the low, slow cooking period.
• 9×13-inch or 3-quart Baking Dish: Standard size for this recipe volume.
• Medium Saucepan: Used exclusively for preparing the béchamel and cheese sauce.
• Wire Whisk: Crucial for making a lump-free roux and béchamel.
• Sharp Knife and Cutting Board: For uniform slicing of the onions and cutting the broccoli.
• Box Grater: For grating the Gruyère and cheddar cheese fresh. This significantly impacts the smoothness of your cheese sauce.
• Spatulas and Wooden Spoons: For constant stirring during the caramelization and gentle folding during assembly.

Detailed Step-by-Step Guide to the French Onion Broccoli Cheese Casserole

Executing this Broccoli Cheese Casserole requires following four distinct phases: Caramelization, Prepping, Sauce Making, and Assembly. Detailed instructions ensure success (800-900 words).

Phase 1: Caramelize the Onions – The Heart of the Flavor

This phase is non-negotiable for creating an authentic French Onion flavor profile.

1. Initial Sweat: Melt 2 tablespoons of butter and the olive oil in your large Dutch oven or deep skillet over medium-low heat. Add the thinly sliced onions. The key here is low, slow heat. Cook for the first 20 minutes, stirring occasionally, until the onions are completely softened, translucent, and have shrunk considerably in volume.
2. Deep Caramelization: Reduce the heat to low. Continue cooking the onions for another 30–40 minutes. You must stir the onions every 5–7 minutes. If any brown bits stick to the bottom, incorporate them; those are the flavor gold. The onions should become deep golden brown and heavily reduced, taking on a jammy texture. Do not rush this. Total cooking time for the onions should be 50–60 minutes.
3. Season and Deglaze: Stir in the dried thyme, kosher salt, pepper, and balsamic vinegar. Cook for 1 minute to allow the vinegar to slightly reduce and infuse. Deglaze the pan by pouring in the 1 cup of rich beef broth. Bring the mixture to a gentle simmer, aggressively scraping up all the browned bits (fond) from the bottom of the pan. This step is essential for maximizing the flavor transfer. Let the broth reduce slightly for 5 minutes until it thickens into a rich, syrupy consistency.
4. Hold: Remove the finished caramelized onion mixture from the heat and set it aside. This forms the flavor base of your French Onion Broccoli Cheese Casserole.

Phase 2: Prepare the Broccoli and Oven

1. Preheat and Prepare: Preheat your oven to 375°F (190°C). Grease a 9×13-inch or 3-quart baking dish thoroughly.
2. Par-Cook the Broccoli: If you are using fresh broccoli, it must be par-cooked (partially cooked) so that it is tender when the casserole finishes baking. Steam or boil the florets for 3–4 minutes only, until they are bright green and slightly tender-crisp. Immediately drain the broccoli and run cold water over it to stop the cooking process. This helps maintain a vibrant color and prevent mushiness. If using frozen broccoli, you skip this step entirely and add it directly, as specified in the ingredient list.

Phase 3: Make the Gruyère Cheese Sauce

This classic béchamel-based sauce needs continuous attention to remain perfectly smooth.

1. Make the Roux: In a medium saucepan, melt the remaining 3 tablespoons of butter over medium heat. Once melted, whisk in the 3 tablespoons of all-purpose flour. Cook this smooth paste, known as the roux, for 1 minute, stirring constantly. Cooking the roux eliminates the raw flour taste.
2. Add Liquid Gradually: Gradually pour in the 1 1/2 cups of half-and-half (or milk), whisking vigorously and continuously as you pour. This ensures the sauce remains perfectly smooth and lump-free.
3. Thicken the Sauce: Bring the liquid to a gentle simmer over medium-low heat, stirring frequently. The sauce will thicken significantly, coating the back of a spoon (nappé consistency). This typically takes about 5–7 minutes.
4. Cheese Integration: Reduce the heat to low. Stir in the 1/4 teaspoon of nutmeg. Remove the sauce from the heat immediately. Stir in the 1 cup of shredded Gruyère cheese and 1 cup of sharp white cheddar cheese, stirring until they are completely melted and the sauce is homogenous and velvety smooth. Avoid boiling the cheese sauce, as this can cause the cheese to separate or become grainy.

Phase 4: Assemble and Bake the French Onion Broccoli Cheese Casserole

1. Combine the Base: Pour the prepared Gruyère cheese sauce into the Dutch oven or skillet containing the caramelized onion mixture. Stir gently until the onion base and the cheese sauce are fully and evenly combined, creating a rich, savory filling.
2. Add Broccoli: Add the par-cooked (or frozen) broccoli florets to the sauce. Gently fold the ingredients together until the broccoli is evenly coated with the rich French Onion Broccoli Cheese Casserole sauce.
3. Transfer and Top: Transfer the mixture to the prepared 9×13-inch baking dish, spreading it into an even layer. In a small bowl, combine the crushed seasoned croutons (or panko) and the extra 1/2 cup of shredded Gruyère cheese. Sprinkle this topping evenly over the entire surface of the casserole.
4. Bake: Bake the casserole in the preheated 375°F (190°C) oven for 25–35 minutes. The casserole is done when the sauce is bubbling visibly around the edges and the topping is deeply golden brown and crisp.
5. Rest and Serve: Remove from the oven and let the casserole rest for 5–10 minutes before serving. This allows the sauce to set slightly, preventing the casserole from running when sliced.

Tips, Variations, and Troubleshooting for Your French Onion Broccoli Cheese Casserole

While the recipe above guarantees a perfect outcome, understanding variations and mastering techniques ensures you can adapt this delicious Broccoli Cheese Casserole to dietary needs or personal preferences (500-600 words).

Essential Tips for Success

• Patience with the Onions: We cannot stress this enough. Caramelization is flavor development. If your onions cook too quickly (turning brown in less than 30 minutes), the heat is too high, and they will taste scorched rather than sweet and deep. Keep the heat genuinely low.
• Freshly Shredded Cheese Only: For the smoothest sauce, avoid pre-shredded cheese. The cellulose used to prevent sticking in pre-shredded bags inhibits smooth melting and can lead to a grainy sauce.
• Managing Frozen Broccoli: If using frozen broccoli, do not increase the baking time drastically, but ensure the casserole rests for the full 10 minutes after baking, as the frozen florets will release more moisture into the sauce. If you want a similar hearty Spanish side dish, check out this great recipe for Empanadas Fáciles, which also benefits from quality ingredients.

Variations and Ingredient Swaps

The beauty of a casserole is its adaptability. Here are ways to modify your French Onion Broccoli Cheese Casserole:

Protein Additions:

To turn this into a full meal, add pre-cooked protein before baking. Shredded rotisserie chicken, leftover ham chunks, or crumbled cooked Italian sausage blend wonderfully with the French onion base.

Vegetable Swaps:

While broccoli is the classic choice, you can substitute or combine it with other vegetables. Cauliflower florets, Brussels sprouts (halved), or sliced green beans all work well. Ensure they are par-cooked slightly before mixing into the sauce.

Cheese Alternatives:

If Gruyère is too expensive or unavailable, a strong Swiss cheese like Emmental, or even a robust Fontina, can provide similar nutty, melting characteristics. You can replace the sharp cheddar with Monterey Jack for a milder, creamier sauce, though the flavor depth will be slightly reduced.

Gluten-Free French Onion Broccoli Cheese Casserole:

Easily adapted! Substitute the all-purpose flour in the roux with an equal amount of certified gluten-free flour blend (ensure it contains xanthan gum) or use cornstarch (you’ll need half the volume of cornstarch mixed with a splash of cold liquid before adding). For the topping, use gluten-free croutons or gluten-free panko breadcrumbs.

Dairy-Free/Vegan Option:

This is trickier due to the French onion base’s reliance on butter and the cheese sauce. Use plant-based butter (like Miyoko’s Kitchen). Replace half-and-half with full-fat oat milk or cashew cream. The cheese can be substituted with vegan Gruyère or vegan sharp cheddar alternatives, though results may vary depending on the quality of the vegan cheese chosen. Substitute the beef broth with high-quality mushroom or vegetable broth.

Troubleshooting Common Casserole Issues

• My Sauce is Grainy: This usually happens if the cheese was added while the sauce was boiling, or if you used pre-shredded cheese. Fix: Try stirring in a tablespoon of cold milk or heavy cream off the heat to emulsify the sauce again.
• My Casserole is Watery: If using frozen broccoli, ensure your par-cooked fresh broccoli was thoroughly drained. If you find the casserole is still too wet after baking, let it rest longer (up to 15 minutes) before serving, or try reducing the amount of beef broth next time.

For more comfort food inspiration and detailed cooking guides, be sure to check out my collection of recipes over at my Pinterest page for Noelia Recipes Blog.

Nutritional Values and Benefits of this Comforting Broccoli Cheese Casserole

While this French Onion Broccoli Cheese Casserole is a decadent comfort dish, it utilizes high-quality ingredients that offer significant nutritional advantages. Analyzing the core components helps understand the benefits of this rich and satisfying meal (400-600 words).

Estimated Nutritional Breakdown (Per Serving, based on 8 servings)

Note: These values are approximations and depend heavily on specific ingredient brands and portion sizes.

• Calories: Approximately 400–450 kcal
• Protein: 18–22g
• Fats: 30–35g (primarily saturated fats from butter and cheese)
• Carbohydrates: 18–25g (includes fiber from broccoli and sugar from caramelized onions/flour)
• Fiber: 3–4g

Benefits of Key Ingredients

Broccoli (The Green Powerhouse)

The foundation of this Broccoli Cheese Casserole is, of course, broccoli. Broccoli is known for being extremely rich in Vitamin C, which supports immune function. It also contains significant amounts of Vitamin K, essential for bone health and blood clotting. Furthermore, broccoli is a good source of fiber, aiding in digestion and promoting gut health. Even when baked into a casserole, its nutritional integrity remains beneficial.

Onions (Antioxidant Richness)

The caramelized yellow onions are more than just flavor providers. Onions are packed with antioxidants, notably quercetin, which has anti-inflammatory properties. While the long cooking process slightly changes the nutrient composition, the onions still contribute essential vitamins and minerals, adding a subtle natural sweetness to the dish without relying heavily on refined sugars.

Gruyère and Sharp Cheddar (Calcium and Protein)

The rich cheese blend contributes substantially to the protein and calcium content of the French Onion Broccoli Cheese Casserole. Dairy is a primary source of calcium, which is vital for maintaining strong bones and teeth. The high protein content makes this casserole incredibly satiating, helping you feel full longer. Specifically, Gruyère, being an alpine cheese, often has a higher concentration of beneficial nutrients due to the density required for its aging process.

Balsamic Vinegar and Beef Broth (Depth and Minerals)

The use of quality beef broth adds mineral content, particularly iron and potentially collagen (depending on the broth type), enhancing the overall richness. Balsamic vinegar, made from cooked grape must, contains trace minerals and antioxidants, offering a depth of flavor that offsets the heavy fats in the cheese sauce, creating a balanced and satisfying profile.

When served as a side, this Broccoli Cheese Casserole provides necessary vegetables and healthy fats to complement a lean protein source. When served as a main dish, it provides robust and balanced macronutrients for a filling comfort meal.

Frequently Asked Questions (FAQs) About French Onion Broccoli Cheese Casserole

We’ve answered some of the most common questions people ask when preparing this decadent French Onion Broccoli Cheese Casserole to ensure your cooking experience is flawless.

Conclusion: The Perfect Marriage of French and Comfort Food

The journey through preparing this French Onion Broccoli Cheese Casserole is one filled with delicious anticipation. From the deep, hypnotic process of caramelizing three large onions an act of patience rewarded with intense flavor to the luxurious, smooth creation of the Gruyère and cheddar béchamel, every step contributes to the ultimate comfort dish. This isn’t just another side; it’s a centerpiece that demands attention and delivers unforgettable results.

We’ve transformed the classic, and sometimes bland, broccoli cheese casserole into a sophisticated dish by integrating the foundational flavors of classic French Onion Soup. The deep umami from the beef broth and balsamic vinegar, combined with the sharpness of the cheddar and the nuttiness of the Gruyère, creates a culinary symphony that complements the tender-crisp broccoli perfectly.

Whether you’re looking for a new holiday staple, a satisfying weeknight meal upgrade, or simply want to try a recipe that showcases layered flavor building, this French Onion Broccoli Cheese Casserole is the answer. Don’t be afraid of the time commitment for the onions; that hour is essential to the dish’s success and will define the savory quality of your final bake.

We encourage you to try this recipe soon. Once you experience the rich, deep flavors that the caramelized onions bring to a simple broccoli bake, you’ll never go back to the basic version. Share your results in the comments below, and let us know how this French Onion Broccoli Cheese Casserole became the star of your table. Happy cooking, and enjoy the savory comfort!

French Onion Broccoli Cheese Casserole

A rich and comforting casserole combining tender broccoli and a creamy Gruyère cheese sauce, infused with the deep, savory, umami flavors of slow-cooked caramelized French onions.

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Prep Time 20 minutes mins

Cook Time 1 hour hr 30 minutes mins

Total Time 1 hour hr 50 minutes mins

Servings: 6 servings

Course: Main Course, Side Dish

Cuisine: American, French Fusion

Calories: 510

Ingredients Equipment Method Notes

Ingredients

  

For the Caramelized Onions (French Onion Base)

• 3 large yellow onions thinly sliced
• 2 tablespoons unsalted butter divided from 4 tbsp total
• 1 tablespoon olive oil
• 1 teaspoon dried thyme
• 1 tablespoon balsamic vinegar
• 1/2 teaspoon kosher salt
• 1/4 teaspoon black pepper
• 1 cup rich beef broth

For the Casserole Filling

• 6 cups broccoli florets fresh or frozen (if frozen, do not thaw)
• 3 tablespoons unsalted butter
• 3 tablespoons all-purpose flour
• 1 1/2 cups half-and-half or whole milk
• 1/4 teaspoon nutmeg
• 1 cup shredded Gruyère cheese
• 1 cup shredded sharp white cheddar cheese

For the Topping

• 1 cup crushed seasoned croutons or panko breadcrumbs
• 1/2 cup extra shredded Gruyère cheese

Equipment

• Large Dutch Oven or Deep Skillet
• 9x13-inch or 3-Quart Baking Dish
• Medium saucepan
• Whisk

Method

 

1. Caramelize the Onions

1. Melt 2 tablespoons of butter and the olive oil in a large Dutch oven or deep skillet over medium-low heat.
2. Add the thinly sliced onions. Cook for 20 minutes, stirring occasionally, until the onions have softened completely.
3. Reduce the heat to low. Continue cooking the onions for another 30–40 minutes, stirring every 5–7 minutes, until they are deep golden brown and heavily caramelized.
4. Stir in the thyme, salt, pepper, and balsamic vinegar. Cook for 1 minute.
5. Deglaze the pan by pouring in the 1 cup of beef broth. Bring to a simmer, scraping up any browned bits from the bottom of the pan. Let the broth reduce slightly for 5 minutes.
6. Remove the caramelized onion mixture from the heat and set aside.

2. Prepare the Broccoli

1. Preheat your oven to 375°F (190°C). Grease a 9x13-inch or 3-quart baking dish.
2. If using fresh broccoli, steam or boil the florets for 3–4 minutes until they are bright green and slightly tender-crisp (par-cooked). Drain thoroughly. If using frozen broccoli, you can add it directly to the casserole mixture without thawing or pre-cooking.

3. Make the Cheese Sauce

1. In a medium saucepan, melt the remaining 3 tablespoons of butter over medium heat.
2. Whisk in the 3 tablespoons of flour to create a smooth roux. Cook for 1 minute, stirring constantly.
3. Gradually pour in the half-and-half (or milk), whisking continuously to prevent lumps. Bring the sauce to a gentle simmer, stirring frequently, until it has thickened enough to coat the back of a spoon (about 5–7 minutes).
4. Reduce the heat to low. Stir in the nutmeg.
5. Remove the sauce from the heat and stir in the 1 cup of Gruyère cheese and 1 cup of sharp cheddar cheese until completely smooth and melted.

4. Assemble the Casserole

1. Pour the prepared cheese sauce into the saucepan containing the caramelized onion mixture. Stir until fully combined.
2. Add the par-cooked or frozen broccoli florets to the sauce. Gently fold the ingredients together until the broccoli is evenly coated.
3. Transfer the mixture to the prepared baking dish, spreading it into an even layer.
4. In a small bowl, combine the crushed croutons (or panko) and the extra 1/2 cup of shredded Gruyère cheese. Sprinkle this topping evenly over the casserole.

5. Bake

1. Bake the casserole in the preheated 375°F (190°C) oven for 25–35 minutes, or until the sauce is bubbly around the edges and the topping is golden brown and crisp.
2. Remove from the oven and let the casserole rest for 5–10 minutes before serving.

Notes

For a deeper, richer flavor, use high-quality beef stock or substitute half of the half-and-half with dry white wine when making the cheese sauce. This casserole can be assembled fully (without the topping) and refrigerated for up to 24 hours before baking; add 10-15 minutes to the bake time if baking from cold.