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 pepper until smooth.
03 - Pour the dressing over the chicken mixture and stir gently until thoroughly combined. Adjust seasoning to taste.
04 - Refrigerate the chicken salad for at least 10 minutes to enhance flavor.
05 - Place a leaf of green leaf lettuce onto 4 slices of bread. Evenly distribute the chicken salad over the lettuce.
06 - Cover with the remaining 4 bread slices to finish each sandwich.
07 - Serve immediately or wrap sandwiches and refrigerate for up to 24 hours.