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