Preheat your oven to 375°F (190°C). Lightly grease a medium baking dish (approximately 9x9 inches or a 1.5-quart oval dish) with non-stick cooking spray or butter.

Prepare the spinach: Ensure the thawed spinach is completely dry. Use a clean kitchen towel or several layers of paper towels to press out as much excess liquid as possible. Excess moisture will result in a watery dip.

In a very large mixing bowl, combine the softened cream cheese, mayonnaise, and sour cream. Use a rubber spatula or a hand mixer on low speed to beat the ingredients until the mixture is completely smooth and no lumps of cream cheese remain.

Add the minced garlic, kosher salt, black pepper, and optional red pepper flakes to the creamy base. Mix thoroughly until the spices are evenly distributed.

Add the thoroughly dried and chopped spinach and the roughly chopped artichoke hearts to the mixing bowl.

Stir in one cup of the Parmesan cheese and one cup of the shredded mozzarella cheese. Reserve the remaining half cup of mozzarella and two tablespoons of Parmesan for the topping.

Gently fold the mixture until the spinach and artichoke hearts are evenly distributed throughout the creamy base. Spoon the prepared dip into the lightly greased baking dish, spreading it evenly.

Sprinkle the reserved half cup of shredded mozzarella cheese and the two reserved tablespoons of Parmesan cheese evenly over the top surface of the dip.

Bake for 25 to 30 minutes, or until the dip is heated through, the edges are bubbly, and the cheese topping is melted, golden brown, and slightly caramelized.

Carefully remove the baking dish from the oven. Allow the dip to cool and settle for about 5 to 10 minutes before serving. This cooling time prevents the dip from being overly runny.

Serve hot directly from the dish with sturdy tortilla chips, slices of toasted baguette, pita chips, crackers, or fresh vegetable sticks.