In a large mixing bowl, combine the cooked chicken, celery, grapes, and red onion.
In a separate small bowl, whisk together the mayonnaise, Dijon mustard, lemon juice, garlic powder, salt, and pepper until smooth.
Pour the dressing over the chicken mixture and stir until everything is well coated.
If using, fold in the chopped parsley for added flavor and color.
Cover the bowl and refrigerate for at least 30 minutes to allow the flavors to meld.
Serve chilled on a bed of lettuce, in a sandwich, or with crackers.