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, 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 the crushed cornflakes with melted butter. Stir until all flakes are evenly coated with butter.
05 - Sprinkle the buttered cornflake mixture evenly over the top of the casserole.
06 - Bake uncovered for 40 to 45 minutes until the topping is golden brown and the casserole is bubbly around the edges.
07 - Let the casserole rest for 5 to 10 minutes before serving to allow it to set for easier scooping.