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 well combined.
Pour the dressing over the chicken mixture and stir gently until everything is evenly coated.
If using, fold in the slivered almonds for added crunch.
Cover the bowl with plastic wrap and refrigerate for at least 30 minutes to allow the flavors to meld.
Serve the chicken salad on a bed of lettuce leaves or as a sandwich filling.