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 all ingredients are evenly distributed.
03 - Spread the hashbrown mixture evenly into the prepared baking dish, pressing lightly to create a uniform layer.
04 - In a separate bowl, toss crushed cornflakes with melted butter until evenly coated.
05 - Sprinkle the buttered cornflakes evenly over the top of the casserole.
06 - Bake uncovered for 40-45 minutes until the top is golden brown and the casserole is bubbly around the edges.
07 - Let the casserole rest for 5-10 minutes before serving to allow it to set.