Old-fashioned Potato Salad

This is a recipe I got from a Barefoot Contessa cookbook, love her recipes!

3 lbs. small red potatoes
kosher salt
1 cup mayonnaise
1/4 cup buttermilk or milk
2 tbsp. Dijon mustard
2 tbsp. whole-grain mustard
1/2 cup chopped fresh dill
freshly ground pepper
1/2 cup medium-diced celery
1/2 cup small-diced red onion

Place the potatoes and 2 tbsp. salt in a large pot of water. Bring the water to a boil, then lower the heat and simmer for 10 - 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 - 20 minutes, until tender but firm.

Meanwhile, in a small bowl, whisk together the mayonnaise, buttermilk, Dijon and whole-grain mustards, dill, 1 tsp. salt and 1 tsp. 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 tsp. salt, and 1 tsp. pepper. Toss well, cover, and refrigerate for a few hours to allow the flavors to blend.