Place the red potatoes in a large pot and cover with cold water by about 1 inch. Add 1 tablespoon of salt to the water.
Bring to a boil over high heat, then reduce to a simmer and cook until the potatoes are fork-tender but still hold their shape, about 15-20 minutes depending on size.
Drain the potatoes thoroughly in a colander and let them cool completely.
Once cooled, dice the potatoes into bite-size cubes, about 3/4-inch pieces. Leave the skins on.
Finely chop the red onion, celery stalks, and dill pickles.
In a large bowl, combine the diced potatoes, chopped red onion, celery, and dill pickles. If using optional add-ins like hard-boiled eggs or fresh herbs, fold them in gently now.
In a separate small bowl, whisk together the mayonnaise, Dijon mustard, apple cider vinegar, sugar, salt, and black pepper until smooth and well combined.
Pour the dressing over the potato mixture and toss very gently with a rubber spatula or wooden spoon.
Cover the bowl and refrigerate for at least 30 minutes, though 2 hours is ideal to allow the flavors to meld together.
Before serving, gently stir the salad again and taste for seasoning. Add a final sprinkle of fresh herbs or paprika on top if desired.
