Prepare the chicken:
Place shredded or cubed chicken in a large mixing bowl.
Add mix-ins:
Add halved grapes, chopped celery, and red onion (if using). For added texture, stir in almonds or walnuts.
Make the dressing:
In a small bowl, whisk together mayonnaise, Greek yogurt, Dijon mustard, lemon juice, salt, and pepper until smooth.
Combine:
Pour the dressing over the chicken mixture. Gently toss until everything is evenly coated.
Chill and serve:
Refrigerate the salad for at least 30 minutes to enhance the flavors. Serve on lettuce leaves, in croissants, or between slices of sandwich bread.