Start by washing and peeling the potatoes with a sharp knife, feeling the smooth skin give way under your blade as you cut into evenly sized chunks.
Place the potato chunks into a large pot filled with cold water, then bring to a boil over medium heat. Let them cook for about 15-20 minutes, until they are just tender when pierced with a fork but still firm enough to hold their shape.
Drain the potatoes using a strainer and transfer them to a bowl of ice water to stop the cooking and cool them down quickly. This also helps keep them firm and prevents over-softening.
While the potatoes cool, thinly slice the red onion and soak the slices in cold water for 10 minutes to mellow their sharpness. Drain and set aside.
In a small bowl, whisk together mayonnaise, Dijon mustard, and apple cider vinegar until smooth and well combined. This will form your creamy dressing.
Gently fold the cooled potatoes into a large mixing bowl, breaking them apart slightly if needed, but keeping most chunks intact for texture.
Add the sliced red onion, chopped celery, and fresh parsley to the bowl, and pour over the dressing. Carefully fold everything together until evenly coated and combined.
Season with salt and pepper to taste, adjusting the flavors as needed. Let the salad sit in the refrigerator for at least an hour to allow the flavors to meld.
Before serving, give the potato salad a gentle toss, taste for seasoning, and add a splash of vinegar or chopped herbs if desired for extra brightness.
Serve the potato salad chilled, garnished with additional parsley if you like, and enjoy the creamy, crunchy bites in every forkful.
