In a large mixing bowl, combine the shredded chicken, chopped celery, and halved grapes.
In a separate small bowl, whisk together the mayonnaise, Dijon mustard, lemon juice, garlic powder, salt, and black pepper until smooth.
Pour the dressing over the chicken mixture and stir until everything is well coated.
If using, fold in the green onions and chopped nuts for added crunch and flavor.
Cover the bowl with plastic wrap and refrigerate for at least 1 hour to allow the flavors to meld.
Serve chilled on a bed of greens, in a sandwich, or with crackers.