Spray a large sheet of heavy duty tin foil (or use non-stick) with cooking spray. Have grill hot or preheat oven to 350 degrees. Wash and dry potatoes well; slice thinly and place on greased foil. (Use a piece large enough to double over top of potatoes).
Sprinkle all dry ingreients over top of potatoes. The amounts may be adjusted according to amount of potatoes used.
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!