Preheat the grill then increase the temperature to 400°F.
Cut and arrange potato slices, separated by onion and butter slices, on a large piece of commercial grade aluminum foil. If commercial grade aluminum foil is unavailable, layer aluminum foil until it is strong, or use a baking sheet.
Top potatoes with garlic, and season with parsley, salt, and pepper. Place potatoes on the aluminum foil.
Place on the preheated grill and cook for 30-40 minutes or until potatoes are tender. Serve hot.
an option, you can sprinkle potatoes with shredded cheddar cheese, reseal foil packets, and continue cooking 5 minutes, or until cheese is melted.