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French Onion Broccoli Cheese Casserole

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
Cook Time 1 hour 30 minutes
Total Time 1 hour 50 minutes
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.