PREHEAT barbecue to medium-high heat. Place potatoes in 2-L microwaveable casserole dish. Add water; cover with lid. Microwave on HIGH 8 min.; drain. Add garlic and 2 Tbsp. of the Italian dressing; toss to coat.
SPRAY large sheet of heavy-duty foil with cooking spray. Spoon potato mixture onto centre of foil. Bring up foil sides. Double fold top and ends to seal packet, leaving room for heat circulation inside.
GRILL 20 min. or until potatoes are tender, turning packet over after 10 min.
MIX Miracle Whip and remaining 2 Tbsp. Italian dressing in large bowl. Add potatoes, bacon and onions; toss lightly. Serve warm.