Slice potatoes as for scalloped potatoes. Slice onions in equal amounts. Arrange alternate slices of onions and potatoes on edge in baking dish, packing slices closely to fill dish.
Top with lots of salt and pepper — more than you think will be enough. Dot with plenty of butter.
Cover tightly with foil and bake at 250° all day, or at least six hours.
Mary said this recipe was perfect for large parties and is good hot or cold.