Peel potatoes and cut into small cube-size pieces. Place potatoes and onions in small saucepan and cover with just enough water to barely cover the potatoes and onion. Cook on medium heat until vegetables are tender. Do not drain off water. Add butter, milk, salt and pepper and heat thoroughly. Remove a small amount of the cooked potatoes and mash with a fork. Return to the pot. This will thicken the liquid a little. Serve hot.