01 - Preheat oven to 350°F. Grease a 9x13-inch baking dish with butter or cooking spray.
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 until evenly distributed throughout.
03 - Spread the hashbrown mixture evenly into the prepared baking dish, pressing gently to create an even layer.
04 - In a separate bowl, toss crushed cornflakes with melted butter until flakes are evenly coated.
05 - Sprinkle buttered cornflakes evenly over the casserole surface. Bake uncovered for 40-45 minutes until topping is golden brown and edges are bubbly.
06 - Remove from oven and let rest 5-10 minutes before serving to allow casserole to set for easier scooping.