Preheat oven to 350°F and grease a 9x13-inch baking dish.
In a large bowl, combine hash browns, cheddar cheese, cream of chicken soup, sour cream, 1/4 cup melted butter, diced onion, pepper, and salt until well mixed.
Spread the mixture evenly into the prepared baking dish.
In a small bowl, mix crushed cornflakes with remaining 1/4 cup melted butter, then sprinkle evenly over the casserole.
Bake for 45 minutes until golden brown and bubbly around the edges.
Let stand for 5 minutes before serving to allow the casserole to set.