Chop the yellow potato into 1-inch cubes and place them in a pot of salted water; bring to a boil and cook until fork-tender, about 10-12 minutes.
Add the egg to the same boiling water during the last 7 minutes of cooking for a hard-boiled finish, then drain and move both to an ice bath.
Season the chicken breast with a pinch of salt and pepper, then grill or pan-sear over medium heat until the internal temperature reaches 165°F.
While the proteins cool, finely dice the celery and thinly slice the radishes into rounds.
In a large mixing bowl, whisk together the Greek yogurt, Dijon mustard, lemon juice, chopped dill, and chives to create the zesty dressing.
Dice the cooled chicken and peel and chop the hard-boiled egg.
Add the potatoes, chicken, egg, celery, and radishes to the dressing bowl and toss gently until everything is evenly coated.
Season with the remaining sea salt and black pepper before serving chilled.