HEAT grill to medium-high heat.
MICROWAVE potatoes and water in 2-qt. covered microwaveable dish on HIGH 8 min.; drain. Add garlic and 2 Tbsp. dressing; toss to coat. Spoon onto large sheet of heavy-duty foil sprayed with cooking spray; fold to make packet.
GRILL 20 min. or until potatoes are tender, turning packet after 10 min.
MIX mayo and remaining dressing in large bowl. Add potatoes, onions and bacon; mix lightly. Serve warm.