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