Place the diced potato in a pot of salted water, bring to a boil, and cook for 10-12 minutes until fork-tender.
While the potato cooks, season the chicken breast with a pinch of salt and pepper, then grill or sauté until fully cooked.
Dice the cooked chicken into bite-sized pieces and set aside to cool slightly.
In a large mixing bowl, whisk together the Greek yogurt, Dijon mustard, apple cider vinegar, salt, and pepper.
Add the cooked potatoes, diced chicken, celery, red onion, dill, and parsley to the bowl.
Gently fold all ingredients together until evenly coated in the dressing and serve chilled.