01 - Preheat oven to 350°F and 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 incorporated.
03 - Spread the hashbrown mixture evenly into the prepared baking dish, pressing gently to create an even layer.
04 - In a separate small bowl, toss the 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 topping is golden brown and the casserole is bubbly around the edges.
06 - Let the casserole rest for 5–10 minutes before serving to allow it to set and make serving easier.