Boil the diced potatoes in salted water for 10–12 minutes, or until fork-tender. Drain and let them cool completely.
Hard-boil the eggs, peel, and chop them into small pieces.
In a large bowl, combine the cooled potatoes, chopped eggs, celery, and red onion.
In a separate bowl, whisk together the mayonnaise, Dijon mustard, and apple cider vinegar until smooth.
Pour the dressing over the potato mixture.
Gently fold everything together until the potatoes are evenly coated without breaking them apart.
Refrigerate for at least 1 hour before serving for the best flavor and creamy texture.