Preheat grill to high heat, about 400°F if your grill has a temperature gauge.
Lay two sheets of heavy duty foil, approximately 24 inches long, on top of each other and form a bowl.
Arrange the potatoes in the center of the bowl. Top the potatoes with the garlic and onion.
Drizzle the potatoes with olive oil.
Place the thyme on top of the potatoes. Season with salt and pepper, then toss to coat.
Seal the foil pouch on the top and sides. Grill for 25-30 minutes over high heat or until the potatoes are fork tender.
Carefully open the foil packet and serve immediately.
Notes
Foil packet can be assembled up to 8 hours in advance and kept at room temperature until ready to grill.
For the oven: Bake the foil packet at 425°F for 25 to 30 minutes, or until the potatoes are fork-tender.
For a foil pan: Add the potatoes, garlic, onion, olive oil, thyme, salt, and pepper to a disposable foil pan. Cover tightly with foil and grill until fork-tender. Larger batches may need additional cooking time.
To prevent sticking: Use enough olive oil to coat the potatoes well, or lightly coat the foil with cooking spray before adding the potatoes.