Ingredients

3 pounds potatoes
Kosher salt
1 cup mayonnaise
1/4 cup buttermilk (or substitute nonfat plain Greek yogurt)
2 tablespoons Dijon mustard
2 tablespoons whole-grain mustard
1/2 cup chopped fresh dill
Freshly ground black pepper
1/2 cup chopped celery
1/2 cup chopped onion

Preparation

1. Place the potatoes and 2 tablespoons of 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 and cover with a clean, dry kitchen towel. Allow the potatoes to steam for 15 to 20 minutes.
2. Meanwhile, in a small bowl, whisk together the mayonnaise, buttermilk (or yogurt), Dijon mustard, whole grain mustard, dill, 1 teaspoon of salt, and 1 teaspoon of pepper. Set aside.
3. When the potatoes are cool enough to handle, roughly chop. Place the cut potatoes in a large bowl. While the potatoes are still warm, pour enough dressing over them to moisten. Add the celery and red onion, 2 teaspoons of salt, and 1 teaspoon of pepper. Toss well, cover, and refrigerate for a few hours to allow the flavors to blend. Serve cold or at room temperature.

From FoodNetwork.com