01 - Preheat oven to 375°F. Lightly 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, blanched broccoli, chopped onion, 1 cup cheddar cheese, cream of chicken soup, sour cream, milk, cooked rice (if using), garlic powder, salt, and pepper until evenly distributed.
04 - Spread the chicken and broccoli mixture evenly in the prepared baking dish, pressing gently to create a uniform layer.
05 - Combine panko breadcrumbs, melted butter, and paprika in a small bowl. Mix until all breadcrumbs are evenly coated.
06 - Sprinkle remaining 1/2 cup cheddar cheese over the casserole. Evenly distribute the buttered breadcrumb mixture on top.
07 - Bake uncovered for 30-35 minutes until the filling is bubbly around the edges and the topping is golden brown and crispy.
08 - Let the casserole rest for 5 minutes before serving to allow the filling to set for easier scooping.