If using uncooked rice, rinse 1 cup of long-grain rice and cook in 2 cups of water until tender, about 15 minutes. Fluff with a fork and set aside.

For pre-cooked rice, measure out 2 cups and keep it warm for later.

In a large skillet over medium heat, melt 1 tablespoon of butter. Add the diced chicken, season with salt and pepper, and sauté until lightly browned, roughly 5 minutes.

Next, introduce the broccoli florets and minced garlic; continue cooking until the broccoli is just tender, approximately 3–4 minutes. Remove from heat and set aside.

In the same skillet, melt the remaining 1 tablespoon of butter over medium heat. Whisk in the flour and cook for 1 minute to create a roux.

Gradually add the chicken broth and heavy cream, stirring continuously until the mixture thickens, about 3–4 minutes.

Stir in the sour cream, onion powder, dried thyme, and a pinch of salt and pepper. Add the cheddar, mozzarella, and Parmesan cheeses, mixing until melted and smooth.

In a large mixing bowl, combine the cooked rice, chicken-broccoli mixture, and cheese sauce, ensuring everything is evenly coated.

Transfer the mixture into a greased 9-x-13-inch baking dish. For a crunchy texture, sprinkle breadcrumbs evenly over the top if desired.

Preheat the oven to 375°F (190°C) and bake the casserole uncovered for 20–25 minutes or until the top is golden brown and the sauce bubbles around the edges.

Allow the casserole to rest for 5 minutes before cutting into servings. Serve warm, optionally garnished with a touch of extra grated Parmesan for added flavor.