In a large mixing bowl, combine the shredded chicken, diced apple, diced celery, halved grapes, and chopped walnuts if using.
In a separate small bowl, whisk together the Greek yogurt, Dijon mustard, honey, lemon juice, salt, and pepper until smooth.
Pour the dressing over the chicken mixture and stir until everything is well coated.
Taste and adjust seasoning if needed, adding more salt, pepper, or lemon juice to your preference.
Chill the salad in the refrigerator for at least 30 minutes to allow the flavors to meld.
Serve the chicken salad on a bed of lettuce leaves or in a sandwich, if desired.