Cook green beans in salted water for about 3 minutes. Remove & cool under cold water. Drain. Cook potatoes 10 minutes or until done. Drain, cool and quarter.
DRESSING: Blend the 1st six ingredients in a blender. Then, with blender running, gradually add olive oil. Blend until thick. Pour 1/2 the dressing over the potatoes and add green beans, tomatoes and onions. Refrigerate salad for 1 hour. Plate salad and add remaining vinegarette dressing.