01 - Preheat oven to 375°F. Grease a 9x13-inch baking dish with cooking spray or butter.
02 - Heat a splash of oil in a large skillet over medium heat. Sauté chopped onion for 3-4 minutes until softened. Add minced garlic and cook 1 additional minute until fragrant.
03 - In a large mixing bowl, combine cooked chicken, broccoli florets, cooked rice, sautéed onion and garlic, 1 1/2 cups cheddar cheese, sour cream, milk, condensed soup, salt, pepper, and paprika. Mix thoroughly until all ingredients are evenly distributed.
04 - Transfer mixture to prepared baking dish, spreading into an even layer. Sprinkle remaining 1/2 cup cheddar cheese over the surface.
05 - In a small bowl, combine panko breadcrumbs with melted butter. Mix until breadcrumbs are evenly coated.
06 - Sprinkle buttered panko breadcrumbs evenly over the cheese layer.
07 - Bake uncovered for 35-40 minutes until casserole is hot and bubbly around edges and topping is golden brown.
08 - Let casserole rest 5-10 minutes before serving to allow flavors to set and make serving easier.