In a large bowl, stir together plain Greek yogurt, dijon, stone-ground mustard, salt, pepper, celery, shallot, and parsley. Mix well until all incorporated.
Add the shredded rotisserie chicken and toss together until chicken is well coated with mixture. If too dry, add more yogurt a tbsp. at a time.
Cover and chill for at least 4 hours before serving.
Serve on a sub roll or between two slices of sandwich bread (or however you want to serve this!)