In a large mixing bowl, combine the diced potatoes, cheddar cheese, sour cream, cream of mushroom soup, chopped onion, salt, and black pepper. Mix well.
Transfer the mixture to a greased baking dish and spread it out evenly.
Bake in the preheated oven for 45 minutes, or until the top is golden brown and the potatoes are tender.
Remove from the oven and let it cool for a few minutes before serving.
Notes:
This casserole is perfect for a cozy family dinner or a potluck gathering.