Clean & cook potatoes and cool. Put green beans in water and boil for 2 to 3 minutes. Then pour off hot water and put beans in ice water to stop cooking. Chop red onion.
When potatoes are cool, slice them and put them in the bottom of serving dish. (Add salt and pepper to potatoes as you layer them.) Put chopped red onion on top of potatoes. Put cooled green beans on top of red onion.
Put all other ingredients in medium bowl and whisk together. Pour over salad. Cover and refrigerate for several hours. Serve cold.