Place hash browns in the bottom of a 9x13-inch baking dish. Stir soup, onion, sour cream, cheese, salt, and pepper together in a medium bowl until well combined; pour over hash browns. Crush cereal and mix with melted butter; sprinkle on top of casserole; cover and let rest in refrigerator 24 hours.

Preheat the oven to 325 degrees F (165 degrees C).

Bake casserole in preheated oven until golden, hot, and bubbly, about 45 minutes.