In a large bowl, whisk together yogurt, mustard, vinegar, honey, ½ teaspoon salt, and ¼ 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.
