01 - Preheat oven to 350°F. Grease a 9x13-inch baking dish with cooking spray or butter.
02 - In a large mixing bowl, combine thawed hashbrowns, shredded cheddar cheese, cream of chicken soup, sour cream, melted butter, chopped onion, garlic powder, salt, and black pepper. Mix thoroughly until all ingredients are evenly distributed.
03 - Spread the hashbrown mixture evenly into the prepared baking dish, pressing lightly to create an even layer.
04 - In a separate bowl, toss crushed cornflakes with melted butter, stirring until flakes are evenly coated.
05 - Sprinkle buttered cornflakes evenly over the casserole. Bake uncovered for 40-45 minutes until topping is golden brown and casserole is bubbly around edges.
06 - Remove from oven and let rest 5-10 minutes before serving to allow casserole to set for easier scooping.