Preheat oven to 375°F (190°C). Grease a 9x13-inch baking dish.
In a large bowl, mix together hashbrowns, cream of chicken soup, sour cream, shredded cheese, melted butter, chopped onion, salt, and pepper.
Spread the mixture evenly into the prepared baking dish.
In a small bowl, combine crushed cornflakes or breadcrumbs with melted butter and sprinkle over the casserole.
Bake for 40-45 minutes or until golden brown and bubbly.
Let cool for 5 minutes before serving. Enjoy!