Pare and cut six large potatoes into dice, or into strips half an inch thick. Leave in cold water for half an hour. Wipe and turn over and over in melted butter until each piece is coated. Pour what remains of the butter into a bake-dish, lay in the potatoes irregularly that the heat may reach all, sprinkle upon them salt, pepper, a few drops of lemon- and the same of onion-juice. Cover the dish and bake, covered, for three-quarters of an hour, or until the dice are tender. Serve dry on a hot dish.