Begin by placing the small white potatoes into a large pot and adding 2 tablespoons of kosher salt. Fill the pot with water and bring it to a boil. Once boiling, reduce the heat and allow the potatoes to simmer for approximately 10 to 15 minutes, or until they are just tender when tested with a knife.

After simmering, drain the potatoes in a colander. To enhance their tenderness, position the colander over the empty pot and cover it with a clean, dry kitchen towel. Let the potatoes steam for about 15 to 20 minutes.

While the potatoes are steaming, prepare the dressing. In a separate small bowl, combine the mayonnaise, buttermilk, Dijon mustard, whole-grain mustard, fresh dill, 1 teaspoon of salt, and 1 teaspoon of freshly ground black pepper. Whisk the mixture until well blended and set aside.

Once the potatoes have cooled sufficiently to handle, cut them into quarters or halves, depending on their size. Transfer the cut potatoes to a large mixing bowl. While still warm, drizzle enough dressing over the potatoes to coat them evenly. Add the chopped celery and red onion, along with 2 teaspoons of salt and an additional teaspoon of black pepper. Toss the ingredients thoroughly to combine, then cover the bowl and refrigerate for several hours to allow the flavors to meld. Serve the potato salad chilled or at room temperature.