Cook the potatoes. Put 3 lbs. red potatoes in a large pot and cover them with water. Add a generous amount of kosher salt, about 2 tablespoons, and bring the water to a boil. Reduce the heat and simmer for about 30 minutes, until the potatoes are tender and a fork slides in easily.
Drain and cool. Strain the potatoes and allow them to cool until they’re comfortable to handle.
Whisk the dressing. While the potatoes cool, whisk 3/4 cup mayonnaise, 1/3 cup buttermilk, 1/4 cup roughly chopped fresh dill, 1 tablespoon Dijon mustard, 1 tablespoon whole grain mustard, and kosher salt and black pepper to taste in the bottom of a large bowl.
Reserve half. Transfer about half of the dressing to a small bowl or container. Cover it and refrigerate it for later.
Cut the potatoes. After the potatoes have cooled for about 10 minutes, cut them into approximately 1-inch pieces.
Combine the salad. Add the potato pieces to the dressing left in the large bowl. Add 2 ribs diced celery and 1/2 cup red onions, then combine.
Chill the salad. Refrigerate for at least 1 hour before serving. You can also refrigerate it overnight.
Add the reserved dressing. Just before serving, mix the refrigerated dressing into the potato salad. Garnish with green onions if desired.
