Chop ¾ of a cup of celery, ¾ cup radishes and ½ cup red onion. Set aside. Pull the tiny fronds from fresh dill stems and set aside 1½ tbsp. of dill fronds.
Place chilled rotisserie chicken on a cutting board and cut the strings. Pick apart the chicken into a mixing bowl, making sure to pick every little piece you can get off the bones. You don’t want to add the skin to your bowl, though. You can have little pieces, but discard whole sections.
Mix ¼ cup mayo, ¼ cup mustard and 1 tsp. white vinegar in a small bowl, and add to the chicken and mix thoroughly. Once your chicken is nicely stirred, add celery, onion, dill, radishes, a sprinkle of salt and pepper, and give it all a good stir.
Depending on how large your rotisserie chicken is, you may require more mayo and mustard to add more moisture to the chicken salad.