2) Drain the potatoes in a colander, then place the colander with the potatoes over the empty pan and cover with a clean, dry kitchen towel. Allow the potatoes to steam for 15 to 20 minutes.
3) Meanwhile, in a small bowl whisk together the mayonnaise, buttermilk, Dijon mustard, whole grain mustard, dill, 1 tsp of salt and 1 tsp of pepper. Set aside.
4) When the potatoes are cool enough to handle, cut them in quarters or in half, depending on their size. Place the cut potatoes in a large bowl and while they are still warm pour over enough dressing to moisten.
5) Add the celery and red onion, 2 tsp of salt and 1 tsp of pepper. Toss well, cover, and refrigerate for a few hours to allow the flavours to blend. Serve cold or at room temperature.