01 - In a large bowl, combine the diced chicken, halved grapes, finely diced celery, minced red onion (if using), and chopped nuts (if using).
02 - In a small bowl, whisk together the mayonnaise, Greek yogurt or sour cream, Dijon mustard, lemon juice, salt, and freshly ground black pepper until smooth and well combined.
03 - Pour the prepared dressing over the chicken mixture and gently toss until all ingredients are evenly coated.
04 - Taste the chicken salad and adjust the seasoning with additional salt or pepper as desired.
05 - Line each sandwich roll, croissant, or bread slice with fresh lettuce leaves, then spoon a generous portion of the chicken salad on top.
06 - Close the sandwiches and serve immediately, or refrigerate the chicken salad mixture for up to two days before final assembly.