01 - Preheat oven to 375°F and grease a 9x13-inch baking dish with cooking spray or butter.
02 - Steam or blanch broccoli florets for 2 to 3 minutes until just tender but still bright green. Drain thoroughly 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, smoothing the top with a spatula.
05 - In a small bowl, combine panko breadcrumbs, melted butter, and paprika. Mix until breadcrumbs 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 to 35 minutes until bubbly around edges and golden brown on top.
08 - Let casserole rest for 5 minutes before serving to allow it to set for easier scooping.