Boil the potatoes whole with the skins on for 20-30 minutes or until done. Drain and cool.
While potatoes cool chop the parsley, tarragon, onion and pepper. Make the dressing: add the lemon juice, garlic, mustard, salt, pepper and olive oil. stir or shake together and set aside.
When potatoes are cool, slice them crosswise into little circles. Stir the herbs and vegetables together and toss with the dressing. Serve immediately or refrigerate until ready to use.