In a bowl, whisk together the onion, mustard, 2 tablespoons of the red wine vinegar, salt and pepper. Gradually whisk in the olive oil and then add the tarragon.
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 or sheet pan to cool slightly. Sprinkle with 1 tablespoon red wine vinegar.
Add some of the vinaigrette to the potatoes and toss; you may not need it all. Add the tomato and toss again. Taste and adjust the seasoning. The salad may need more salt.