01 - In a large mixing bowl, combine the diced chicken, halved grapes, celery, minced red onion, and chopped nuts.
02 - In a small mixing bowl, whisk together mayonnaise, Greek yogurt, Dijon mustard, lemon juice, salt, and black pepper until smooth and fully incorporated.
03 - Pour the prepared dressing over the chicken mixture and gently toss until all ingredients are evenly coated with dressing.
04 - Taste the chicken salad and adjust seasoning with additional salt and pepper as needed.
05 - Line each bread roll or slice with lettuce leaves, then spoon the chicken salad evenly onto the lettuce.
06 - Close the sandwiches and serve immediately, or refrigerate the chicken salad for up to two days before assembly.