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 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 bowl, toss crushed cornflakes with melted butter until evenly coated.
05 - Sprinkle the cornflake mixture evenly over the casserole. Bake uncovered for 40 to 45 minutes, or until the top is golden brown and the casserole is bubbly around the edges.
06 - Remove from oven and let rest for 5 to 10 minutes before serving to allow the casserole to set for easier scooping.