Place the potatoes in a large pot and fill with cold water to 1-inch above the potatoes. Season the water with 1 tablespoon salt. Bring to a simmer and cook until a knife gently plunges through the potatoes, about 15 minutes. Reserve 1/3 cup of the cooking water, drain the potatoes and put back into the pot (off the heat) to keep warm.
Meanwhile, fry the bacon in a skillet over medium heat until crispy, stirring occasionally. Reserve the bacon and drain all but 1/4 cup bacon fat. Add the onions and saute until slightly golden. Add the vinegar and, with a wooden spoon, scrape up any bits from the bottom of the skillet. Add in the reserved cooking water and reduce until it thickens, about 3 minutes. Off the heat, whisk in the mustard and season with salt and pepper. Add in the potatoes, bacon and tarragon and toss to combine. Adjust the seasoning if necessary.
Recipe courtesy of Jeff Mauro