In a large mixing bowl, combine the diced potatoes, cream of celery soup, shredded cheddar cheese, sour cream, 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.
Let the casserole cool for a few minutes before serving.
Notes:
This casserole is a great side dish for any meal or can be enjoyed on its own.