Preheat oven to 375 degrees F or 190 degrees C. Throughly blend soup and milk in a bowl. In a greased 3 qt (3L) casserole dish, layer one half of each of the following; potatoes, onion mushrooms, cheese and soup mixture, ending with the soup mixture. Repeat layers. Top with dots of butter or margarine.

Cover and bake one hour at 375 degrees F or 190 degrees C. remove cover and continue baking 30 minutes more, or until potatoes are tender.