Chop the red potato into bite-sized cubes and boil in a pot of salted water until fork-tender, then drain and let cool completely.
Season the chicken breast with a pinch of salt and pepper, then grill or pan-sear until the internal temperature reaches 165°F.
Once the chicken is cooled, dice it into small cubes to match the size of the potatoes.
In a large mixing bowl, whisk together the Greek yogurt, Dijon mustard, lemon juice, sea salt, and black pepper until smooth.
Add the cooled potatoes, diced chicken, celery, red onion, and fresh dill to the bowl.
Gently fold all ingredients together until everything is evenly coated in the creamy dressing.
Serve immediately at room temperature or chill in the refrigerator for 30 minutes for a colder salad.