PREHEAT oven to 400°F. Toss potatoes with 3 Tbsp. of the dressing; spread into shallow foil-lined baking 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. Add potatoes, celery, walnuts and onions; toss lightly. Serve immediately. Or, cover and refrigerate until ready to serve.
