01 - Preheat the oven to 375°F. Lightly grease a 9x13-inch baking dish with cooking spray or butter.
02 - Heat a large skillet over medium heat. Add a splash of oil and sauté the chopped onion for 3–4 minutes until softened. Add the minced garlic and cook for an additional minute until fragrant.
03 - In a large mixing bowl, combine the cooked chicken, broccoli florets, cooked rice, sautéed onion and garlic, 1½ cups of the shredded cheddar cheese, sour cream, milk, condensed cream of chicken soup, salt, black pepper, and paprika. Mix thoroughly until all ingredients are evenly distributed.
04 - Spread the mixture evenly into the prepared 9x13-inch baking dish, smoothing the top with a spatula.
05 - Sprinkle the remaining ½ cup of shredded cheddar cheese evenly over the top of the casserole.
06 - In a small bowl, combine the panko breadcrumbs with the melted butter. Mix until the breadcrumbs are evenly coated.
07 - Sprinkle the buttered panko breadcrumbs evenly over the cheese layer. Bake uncovered for 35–40 minutes until the casserole is hot and bubbly around the edges and the topping is golden brown.
08 - Remove from the oven and let the casserole rest for 5–10 minutes before serving to allow it to set.