PREHEAT barbecue to medium heat. Spray large sheet of heavy-duty foil generously with cooking spray; set aside. Mix dressing and garlic.
PLACE potatoes in centre of foil; top with onions. Drizzle with half of the dressing mixture. Bring up foil sides. Double fold top and both ends to seal packet, leaving room for heat circulation inside.
GRILL 30 min. Carefully open foil packet; turn potatoes over. Grill, in opened packet, an additional 5 min. or until potatoes are tender. Spoon into serving dish; drizzle with the remaining dressing mixture.