Step 1

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.

Step 5

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.

Step 29

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.