- 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.