Preheat your oven to 375°F (190°C).
In a large bowl, combine the cooked chicken, rice, broccoli, cream of chicken soup, sour cream, 1 cup cheddar cheese, garlic powder, pepper, salt, and milk. Mix until well blended.
Pour the mixture into a greased 9x13-inch baking dish and spread evenly.
Top with the remaining 1/2 cup of shredded cheese and optional crushed crackers or breadcrumbs for extra crunch.
Bake for 25–30 minutes, or until the casserole is hot and bubbly and the top is golden.
Let it sit for 5 minutes before serving.