COOK potatoes in boiling water in saucepan 20 min. or until tender; drain.
HEAT grill to medium-high heat. Place potatoes in large bowl. Add 2 Tbsp. dressing; toss to coat. Place potatoes and onions on grate of grill. Brush onions with 1 Tbsp. of remaining dressing.
GRILL 10 min. or until browned, turning occasionally and brushing onions with 1 Tbsp. of remaining dressing. Place potatoes and onions in serving bowl. Add remaining dressing; toss to coat. Serve warm.