In a large mixing bowl, combine the shredded chicken, celery, and red grapes.
In a separate bowl, whisk together the mayonnaise, Greek yogurt, 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 slivered almonds and green onions for added crunch and flavor.
Cover the bowl with plastic wrap 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.