Spray a 9x13" baking dish with non-stick cooking spray and set aside.
In a large skillet, melt butter over medium high heat. Stir in garlic and cook for 30 seconds. Add in flour, garlic powder, salt, and pepper and mix together using a whisk.
Pour in chicken broth, whisking constantly, until the sauce begins to boil and thicken.
Stir in 2 1/2 cups shredded cheese until melted and smooth.
Add in the cooked rice, cooked chicken, and cooked broccoli. Gently fold all together.
Carefully dump the chicken broccoli rice mixture into the prepared pan.
Top with remaining 1 cup of cheese and fresh parsley, if using.
Bake for 20 minutes or until the cheese is melted.
Notes
When you are making this recipe, yes it is a good idea to have it made up in advance or cook it while you are making the cheesy base for the casserole.