Cut butter or margarine into small pieces and place over potatoes. place onion slices over top of potatoes. Fold foil over potatoes, and roll top ends of foil over to seal, and do the same with both sides to make a secure packet. Place on grill or in oven and cook until soft. (With glove on, lightly press to check for doneness). Be careful when opening, as the foil and steam will be very hot!