Preheat grill for high heat.
Arrange potato slices, separated by onion and butter slices, on a large piece
of aluminum foil. Top with garlic, and season with parsley, salt, and pepper.
Tightly seal potatoes in the foil.
Place on the preheated grill, and cook 20 minutes, turning once, or until
potatoes are tender.
Sprinkle potatoes with Cheddar cheese, reseal foil packets, and continue
cooking 5 minutes, until cheese is melted.