Preheat oven to 375-400 degrees. I love the potatoes and they are even better the next day so I usually make a large batch. Make sure the onions and potatoes are sliced as thinnly as you can get them. A mandolin helps. Mix soup and milk together. Spray your casserole dish well. Layer in the following order and repeat until all items gone: Potatoes, Onions, Salt, Pepper, Cheese, soup mixture. Top with more cheese. Bake until fork tender. These will take QUITE a while so be prepared for 1 1/2 to 2 hours but they are well worth the wait!!!