Grease a large square of heavy aluminum foil. Arrange sliced potatoes on foil, sprinkle with salt and pepper and cover with sliced onions. Add pats of butter or margarine. Wrap and seal foil. Cook over hot coals on a grill until done (30 or 40 minutes depending on fire). Open foil and add Cheddar strips. Cover again and grill for a couple of minutes, just until cheese melts.