Heat the soup and stir in the milk, salt and pepper, and onion.
In a greased medium casserole make alternate layers of potatoes and soup.
Dot each layer with a little butter.
At the end pour any leftover soup over the casserole and use any remaining butter to dot the top.
Cover and bake 30 minutes in a 375° oven.
Remove the cover for the last 15 minutes.