Place the diced potatoes in a large pot and cover with cold water. Bring to a boil over medium-high heat and cook for about 10-15 minutes, or until the potatoes are tender but not mushy. Drain and let cool completely.
In a large mixing bowl, combine the mayonnaise, Dijon mustard, apple cider vinegar, salt, and black pepper. Mix well until smooth.
Once the potatoes have cooled, add them to the bowl with the dressing. Gently fold in the celery, red onion, hard-boiled eggs, and parsley until everything is well coated.
Cover the potato salad and refrigerate for at least 1 hour before serving to allow the flavors to meld.
Serve chilled and enjoy!