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 and allow to cool.
Meanwhile, in a small bowl, whisk together the mayonnaise, buttermilk, Dijon mustard, whole grain mustard, dill, 1 teaspoon of salt, and 1 teaspoon of pepper. Set aside.
When the potatoes are cool enough to touch, cut them in quarters or in half, depending on their size. Place the cut potatoes in a large bowl. While the potatoes are still warm, pour the dressing over and add the celery, 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.
Recipe by Big City Tiny Kitchen at https://bigcitytinykitchen.com/red-potato-and-dill-salad/