Directions
In a large bowl, combine the cooked chicken, celery, red onion, grapes, and chopped walnuts. In a small bowl, stir together the mayonnaise, Dijon mustard, lemon juice, salt, and black pepper. Pour the dressing over the chicken mixture and stir until evenly combined. Place the lettuce leaves on 4 bread slices. Divide the chicken salad among the lettuce-covered slices and top with the remaining bread slices. Cut each sandwich in half before serving.