Cook and stir onions in butter in large saucepan 5 minutes 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 teaspoon each chopped fresh cilantro and finely chopped red onions.