In a large bowl, whisk together yogurt, mustard, vinegar, honey, 1/2 teaspoon salt, and 1/4 teaspoon pepper. Add chicken and toss to coat. Fold in celery, apple, and grapes.  Serve on lettuce leaves and sprinkle with walnuts. Make a wrap: Toss the walnuts into the chicken mixture, then layer a whole-grain tortilla with lettuce leaves and 1 cup chicken-apple-walnut-salad.