01 - Preheat oven to 350°F. Grease a 9x13-inch baking dish with cooking spray or butter.
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 incorporated.
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 evenly coated.
05 - Sprinkle the buttered cornflake mixture evenly over the top of the casserole.
06 - Bake uncovered for 40-45 minutes until the topping 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 for easier scooping.