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, condensed 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 gently to create a uniform layer.
04 - In a separate small bowl, toss crushed cornflakes with melted butter until flakes are evenly coated.
05 - Sprinkle buttered cornflakes evenly over the casserole surface. Bake uncovered for 40 to 45 minutes until topping is golden brown and casserole is bubbly around edges.
06 - Remove from oven and let rest for 5 to 10 minutes to allow casserole to set for easier serving.