Place hash browns in the bottom of a 9x13 inch baking dish. In a medium bowl, stir together soup, onion, sour cream, cheese, salt and pepper until well combined. Pour over hash browns. Crush the cereal and mix with the butter. Sprinkle mixture over soup layer. Cover and let rest in refrigerator 24 hours.
Preheat oven to 325 degrees F (165 degrees C).
Bake in preheated oven 45 minutes, until golden, hot and bubbly.