Begin by placing the scrubbed and chunked potatoes into a large pot filled with salted water. Bring the water to a rolling boil and cook the potatoes for about 10-15 minutes, until they are fork-tender but still firm. Drain them well in a colander and let them cool slightly while you prepare the other ingredients.

While the potatoes are cooling, wash the mustard greens thoroughly under cold water to remove any grit. Remove any tough stems, then chop the leaves into bite-sized pieces. Set aside the chopped mustard greens in a bowl.

In a medium bowl, combine the mayonnaise, Dijon mustard, apple cider vinegar, olive oil, salt, and pepper. Using a whisk, mix everything together until smooth and creamy.

In a large mixing bowl, add the slightly cooled potatoes, chopped mustard greens, thinly sliced red onion, and diced celery. Gently fold the ingredients together with a spatula until evenly combined.

Pour the creamy dressing over the potato mixture and toss everything together until all the ingredients are well-coated.

Transfer the finished salad to a serving bowl and garnish with freshly chopped parsley or dill. Cover the bowl with plastic wrap or a lid and refrigerate for at least 30 minutes.