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, condensed 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 bowl, toss the crushed cornflakes with melted butter until evenly coated and the butter is distributed throughout.
05 - Sprinkle the buttered cornflake mixture evenly over the entire surface of the casserole.
06 - Bake uncovered for 40–45 minutes, or 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 for easier scooping.