Transfer the potatoes, onion and garlic to a large bowl.
Drizzle with the oil and season with salt and pepper. Toss to combine and coat.
Tear off a large piece of aluminum foil.
Place the potato mixture in the center of the foil and fold up the edges to seal.
Transfer to a grill and cook 15-20 minutes or until tender. To test, you can poke the potatoes with a thin skewer. Once the potatoes are as tender as you like, remove the packets from the grill.
Carefully open the packets (the hot steam released might burn you) and serve.