Preheat Oven: Set your oven to 350°F (175°C).
Prepare Broccoli: Steam or blanch the broccoli florets until they're just tender, then arrange them in a greased 9x13-inch baking dish.
Layer Chicken: Place the cooked chicken slices evenly over the broccoli.
Make the Sauce:
In a saucepan over medium heat, melt the butter.
Whisk in the flour to form a roux, cooking for about 2 minutes.
Gradually add the chicken broth, heavy cream, and dry sherry, whisking continuously until the mixture thickens.
Stir in the garlic powder, salt, black pepper, and half of the Parmesan cheese.
Assemble the Casserole:
Pour the sauce evenly over the chicken and broccoli.
Sprinkle the remaining Parmesan cheese, shredded cheddar cheese, and breadcrumbs on top.
Bake: Place the casserole in the preheated oven and bake for 25–30 minutes, or until the top is golden brown and the sauce is bubbly.
Serve: Allow the dish to cool for a few minutes before serving. Enjoy your homemade Chicken Divan!