COOK and stir onions in butter in large saucepan 5 min. or until tender. Add broth and potatoes; simmer 15 to 20 min. or until potatoes are tender. Transfer to blender; cover. Blend until smooth. Place in large bowl.
ADD dressing, milk and cucumber to blender; cover. Blend until smooth. Add to potato mixture; stir until blended. Season to taste with pepper, if desired; cover.
REFRIGERATE at least 4 hours. Garnish each serving with 1 tsp. each chopped fresh cilantro and finely chopped red onions.