1. Using the slicing blade on the food processor, lock the lid into place and push the shallots and beans down the feed chute with the pusher, until all have been thinly sliced.
  2. In a saucepan cover the potatoes with cold water and simmer over a low heat for 25 minutes. Drain and allow to cool for 10 minutes.
  3. In a large serving bowl, toss together the shallots, green beans, mayonnaise, mustard, salt and potatoes until just combined. Serve immediately.