Scrub potatoes; leave whole and place in a large pot. Cover with cold water so that there's at least an inch of water above the potatoes. Add 2 tablespoons salt and place over medium-high heat. Bring to a boil, reduce heat to low, and simmer until tender, about 20 to 25 minutes.

Put diced onion and minced garlic in a large bowl with vinegar, remaining salt, and black pepper. Stir to dissolve salt; then whisk in olive oil. Set aside.

When potatoes are done, drain and cut into 1-inch cubes. While potatoes are still warm, toss with onion mixture, mustard, and capers. Add bell pepper, fennel, celery (or lovage), and herbs and toss once more. Serve.