You can serve this salad warm or cold but don’t add the green beans until just before serving to conserve their bright color.
Bring a large pot of salted water to a boil.
Scrub 1 lb. of potatoes. Trim 1/2 lb. of green beans. Trim 1 leek and then slice the shank in half lengthwise and run it under water to clean.
Cut the leek lengthwise into thin slices.
Heat 2 T. olive oil in a small skillet and add the leeks plus salt and pepper. Saute on low heat until the leeks are soft and beginning to brown. Turn off the heat.
Drop the green beans into the water and stir to cover. When the water comes back to a boil, skim them out with a slotted spoon. Rinse to cool an drain them.
Add the potatoes to the water and cook until they are just fork-tender. Drain the pot, then cover the potatoes with cold water to cool. When you can pick them up, slice them into green-bean sized pieces.
Add 3 T. white wine vinegar or lemon juice to the pan with leeks. Stir it well to deglaze the pan, then transfer to a small bowl and mix in 1 T. Dijon mustard. Taste the dressing and add more salt and pepper if need be.
Toss the potatoes with the leek dressing. If you are serving the salad immediately, add the green beans as well. If not, add them 5 minutes before serving. Serve over a bed of washed Arugula or Spinach leaves.