Preheat oven.
Use a mandoline to cut the potatoes and onions. Put them in separate bowls.
Mix the soup and milk together with a whisk in another bowl.
In a glass baking dish, line with pads of butter. A layer of potatoes then onions then milk mixture, top with salt and pepper. Layer the same way about 3 more times or until you run out of ingredients (add more milk to the soup mixture if you need to).
Line a cookie sheet that is larger than the potato dish with aluminum foil. Cover the potato dish with aluminum foil.
Bake at 350 for 1.5 to 2 hrs. Or 400 for 1 to 1.5 hrs.
When the dish is close to done, take the foil off the potato dish and cook for another 15 minutes to get the top bubbly.
Serve with ham and enjoy!