HEAT oven to 400°F.
TOSS potatoes with 3 Tbsp. dressing; spread onto bottom of shallow foil-lined pan.
BAKE 25 to 30 min. or until potatoes are tender, stirring after 15 min. Cool completely.
MIX mayo and remaining dressing in large bowl until blended. Add potatoes, celery, nuts and onions; toss to evenly coat.