Place foil packets on the counter and spray with cooking spray.
Wash and slice potatoes (and onions/mushrooms if using).
Place potato slices (approx 1 potato per packet) and any optional toppings on each foil piece. Top with a generous pat of butter and salt/pepper to taste. Fold each packet closed.
Grill over medium heat for 30 minutes or until soft.
(Nutrition information provided is an estimate and will vary based on cooking methods and brands of ingredients used.)