PREHEAT barbecue to medium-high heat. Place potatoes in 2 L microwaveable baking dish. Add water; cover. Microwave on HIGH 8 min., rotating dish after 4 min. Meanwhile, cut 1/3-inch off top of the garlic head; discard top. (Do not cut root end.) Place on sheet of foil; drizzle with 1 Tbsp.of the dressing. Wrap garlic in foil, sealing ends tightly. Drain potatoes.

THREAD potatoes onto skewers. Brush with some of the remaining dressing. Grill potatoes and garlic 10 to 15 min. or until potatoes are tender, turning the potatoes frequently and brushing with remaining dressing. Remove potatoes and garlic from grill; cool.

REMOVE garlic pulp from skins (see Tip); mix with mayo and mustard in large bowl. Add potatoes and celery; mix lightly. Cover. Refrigerate several hours or until chilled.