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 thoroughly until all ingredients are evenly distributed.
03 - Spread the hashbrown mixture evenly into the prepared baking dish, pressing lightly 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 cornflake mixture 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 - Remove from oven and let rest for 5–10 minutes before serving to allow the casserole to set.