01 - If using raw chicken, place breasts in a saucepan and cover with water. Add a pinch of salt and simmer gently until cooked through, approximately 15-20 minutes. Allow to cool completely, then dice or shred into bite-sized pieces.
02 - In a large mixing bowl, combine diced chicken, halved grapes, diced celery, toasted walnuts, red onion, and fresh parsley. Mix gently to distribute ingredients evenly.
03 - In a separate small bowl, whisk together mayonnaise, Greek yogurt, Dijon mustard, and lemon juice. Season with salt and freshly ground black pepper to taste.
04 - Pour prepared dressing over the chicken mixture and toss gently until all components are evenly coated. Taste and adjust seasoning as needed.
05 - Toast bread slices lightly if desired to add texture and prevent sogginess.
06 - Lay out 4 bread slices. Top each with a layer of mixed salad greens, then spoon generous portions of chicken salad over the greens. Cover with remaining bread slices.
07 - Cut assembled sandwiches in half diagonally and serve immediately while bread is still warm and fresh.