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 thoroughly to remove excess moisture.
03 - In a large mixing bowl, combine cooked chicken, blanched broccoli, chopped 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 the mixture evenly in the prepared 9x13-inch baking dish, pressing gently to create an even layer.
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 evenly over the casserole mixture. Top with buttered breadcrumb mixture, spreading to cover the surface.
07 - Bake uncovered for 30–35 minutes, or until the casserole is bubbling around the edges and the topping is golden brown.
08 - Remove from oven and let the casserole rest for 5 minutes before serving to allow the filling to set.