01 - Preheat oven to 375°F. Grease a 9x13-inch baking dish with cooking spray or butter.
02 - Steam or blanch broccoli florets for 2-3 minutes until tender-crisp and bright green. Drain thoroughly to remove excess moisture.
03 - In a large bowl, mix cooked chicken, broccoli, onion, 1 cup cheddar cheese, cream of chicken soup, sour cream, milk, cooked rice (if using), garlic powder, salt, and pepper until evenly combined.
04 - Transfer mixture to prepared baking dish and spread in an even layer.
05 - In a small bowl, combine panko breadcrumbs with melted butter and paprika, mixing until crumbs are evenly coated.
06 - Sprinkle remaining 1/2 cup cheddar cheese over the casserole, then evenly distribute the buttered breadcrumb mixture on top.
07 - Bake uncovered for 30-35 minutes until sauce is bubbly and topping is golden brown.
08 - Let casserole rest for 5 minutes before serving to allow flavors to set.