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 just tender but still bright green. Drain well to remove excess moisture.
03 - In a large bowl, combine cooked chicken, broccoli, onion, 1 cup cheddar cheese, cream of chicken soup, sour cream, milk, cooked rice (if using), garlic powder, salt, and pepper. Mix until evenly distributed.
04 - Spread mixture evenly in the prepared baking dish, pressing lightly to create an even layer.
05 - In a small bowl, mix panko breadcrumbs with melted butter and paprika until evenly coated.
06 - Sprinkle remaining 1/2 cup cheddar cheese over the casserole, then top with the buttered breadcrumb mixture.
07 - Bake uncovered for 30–35 minutes, or until bubbly and golden brown on top.
08 - Let casserole rest for 5 minutes before serving to allow it to set.