Place the cubed red potatoes in a pot of salted water, bring to a boil, and cook until fork-tender, about 10-12 minutes.
While potatoes cook, season the chicken breast with a pinch of salt and pepper, then grill or pan-sear until the internal temperature reaches 165°F.
Let the chicken rest for 5 minutes before dicing it into bite-sized pieces.
In a large mixing bowl, whisk together the Greek yogurt, Dijon mustard, lemon juice, chopped dill, sea salt, and black pepper until smooth.
Add the drained potatoes, diced chicken, celery, and red onion to the bowl with the dressing.
Gently fold all ingredients together until evenly coated in the creamy sauce.
Serve immediately or chill in the refrigerator for 30 minutes to allow the flavors to meld.