Preheat an outdoor grill for medium heat.

Cut aluminum foil into 2 or 3 squares large enough to easily wrap vegetables. Divide and place potatoes and onion in centers of each foil square; season with salt and black pepper and dot with butter. Wrap each foil square into a flattened square and seal the edges.

Place aluminum-wrapped packages over indirect heat; cover the grill. Cook on the preheated grill for about 30 minutes, flipping once.