cut a long piece of extra-wide foil about 2 ft., long. If you don't have wide foil you can take two regular width foil pieces and fold the long edges together twice to make one wide piece. You will need two of these wide pieces.
slice the potatoes (with the skin on) and the onion into thin slices
Spray one of the foil pieces (non-shiny side) with cooking spray. Put a layer of potatoes (overlapping a bit) onto the center of one of the pieces of foil, top with some onion rings, 4 -1/3 tbsp of butter pads, and salt and pepper, continue this with two more layers of the same.
Fold up the foil that the potatoes are sitting on to make a pack and then use the other piece to seal up the pack, making sure that the seams are opposite with each pack. Make sure all the foil edges are firmly gathered and sealed. You want to steam the potatoes in the foil pack and not let any of the moisture out.
Pop the foil pack on a baking sheet and put it into a 350° oven for about an hour. If you put it on the grill, you can just lay it on the grates.
Be careful when you are opening the packs as hot steam will come out of them.
Notes
Add cheese and/or other spices as desired. Pretty much everything in the pack is "to taste"