Place the potatoes and 2 tablespoons salt in a large pot of water. Bring the water to a boil, then lower the heat and simmer for 10 to 15 minutes, until the potatoes are barely tender when pierced with a knife. Drain the potatoes in a colander, then place the colander with the potatoes over the empty pot off the heat and cover with a clean, dry kitchen towel. Leave the potatoes to steam for 15 to 20 minutes, until tender but firm.
Meanwhile, in a small bowl, whisk together the mayonnaise, buttermilk, Dijon and whole-grain mustards, dill, 1 teaspoon salt, and 1 teaspoon pepper. Set aside.
When the potatoes are cool enough to handle, cut them into quarters or halves, depending on their size. Place the cut potatoes in a large bowl and pour enough dressing over them to moisten. (As the salad sits, you may need to add more dressing.) Add the celery and red onion, 2 teaspoons salt and 1 teaspoon pepper. Toss well, cover, and refrigerate for a few hours to allow the flavors to blend.
Copyright 2006, Barefoot Contessa at Home by Ina Garten, Clarkson Potter/Publishers, All Rights Reserved