Preheat your oven to 375°F (190°C). Grease a 9×13 inch baking dish.
Cook the rice according to package directions. While the rice cooks, bring a pot of salted water to a boil. Add the broccoli florets and blanch for 1-2 minutes. Drain immediately and set aside.
In a large saucepan, melt 4 tbsp of butter over medium heat. Whisk in the flour and cook for 1 minute, stirring constantly, to form a roux.
Gradually pour in the milk, whisking continuously to ensure the sauce is smooth. Continue cooking and stirring for 5-7 minutes, until the sauce has thickened enough to coat the back of a spoon.
Remove the saucepan from the heat. Stir in the garlic powder, onion powder, salt, and pepper. Add 2.5 cups of the shredded cheddar cheese and stir until melted and smooth.
Add the cooked rice and blanched broccoli to the cheese sauce. Fold gently until everything is well combined.
Pour the mixture into the prepared baking dish and spread evenly.
In a small bowl, combine the Panko breadcrumbs, Parmesan cheese, and 2 tbsp of melted butter. Sprinkle this mixture evenly over the casserole, followed by the remaining 0.5 cup of cheddar cheese.
Bake for 20-25 minutes, or until the casserole is hot and bubbly and the topping is golden brown. Let it rest for 5-10 minutes before serving.
Notes
For a creamier texture, substitute half-and-half for whole milk. This dish can also be made gluten-free by using gluten-free flour.