01 - In a large bowl, mix chicken, grapes, celery, red onion, and parsley until evenly distributed.
02 - In a small bowl, whisk together mayonnaise, Greek yogurt if using, Dijon mustard, lemon juice, salt, and black pepper until smooth.
03 - Pour the dressing over the chicken mixture and gently fold until fully incorporated. Taste and adjust seasoning as needed.
04 - Refrigerate the chicken salad for at least 10 minutes to enhance the flavors, if desired.
05 - Place one lettuce leaf on each of four bread slices. Evenly spoon the chicken salad over the lettuce.
06 - Top with remaining bread slices to close sandwiches.
07 - Serve immediately or wrap sandwiches and refrigerate for up to 24 hours.