1. Boil potatoes in their skins, covered, just until they can be easily pierced with a sharp knife.
2. Meanwhile, prepare dressing in a jar, combining mustard, vermouth, vinegar, olive oil, salt and pepper to taste; shake well.
3. Drain potatoes, let cool enough to handle, then peel and cut into thick slices. Place in a large bowl.
4. Pour dressing over the potatoes while they are warm, tossing well.
5. Add chopped onion, sliced celery, capers, finely chopped parsley, dill and, if you like, other chopped vegetables (red bell pepper, radish).
6. Correct seasoning. Chill until served.
7. If desired, you can toss in lightly cooked fresh green beans as a good last-minute addition.
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