Preheat oven to 350 degrees. Peel potatoes and cut lengthwise in halves or quarters, then slice thinly. Use a food processor to make it really easy. Place in cold water for 10 minutes, drain thoroughly.
Arrange potatoes in oblong glass baking dish that holds them well, not too spread out. You want a couple of layers. Season with salt and pepper. Dot with butter.
Stir soup and milk together until well-blended. Pour evenly over potatoes. Bake for 45 minutes (or longer) until done to your liking, potatoes being soft and sauce thick and browned a little on top.
This can also be done in a crock pot. I prefer high power for 3-4 hours.