01 - Preheat oven to 375°F. Lightly grease a 9×13-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 translucent. Add minced garlic and cook for 1 minute until fragrant. Remove from heat.
03 - In a large mixing bowl, combine cooked chicken, broccoli florets, cooked rice, sautéed onion and garlic, 1½ cups cheddar cheese, sour cream, milk, condensed cream of chicken soup, salt, pepper, and paprika. Mix thoroughly until all ingredients are evenly incorporated.
04 - Transfer the mixture to the prepared baking dish, spreading it into an even layer with a spatula.
05 - Sprinkle the remaining ½ cup cheddar cheese evenly over the top of the casserole.
06 - In a small bowl, combine panko breadcrumbs with melted butter. Mix until evenly coated.
07 - Sprinkle the buttered breadcrumbs evenly over the cheese layer. Bake uncovered for 35–40 minutes until the casserole is hot and bubbly and the topping is golden brown.
08 - Remove from oven and let the casserole rest for 5–10 minutes before serving to allow the filling to set.