Make the vinaigrette: In a bowl, whisk together the mustard, vinegar, and salt. Whisk in the olive oil gradually, then season with pepper. Put the potatoes in a large pot of cold, well-salted water to cover. Bring to a boil, adjust the heat to maintain a gentle simmer, and cook until tender when pierced, about 15 minutes. Drain well and transfer to a large bowl to cool slightly. Add some of the vinaigrette to the potatoes and toss; you may not need it all. Add the tomato, onion, and tarragon and toss again. Taste and adjust the seasoning. The salad will probably need more salt. |