1Cook and drain pasta as directed on package. Rinse with cold water; drain.
2In very large (4-quart) bowl, mix pasta, chicken, grapes, celery and onion. In small bowl, mix oil and 1 tablespoon of the fresh thyme (or 1 teaspoon of the dried thyme). Pour oil mixture over chicken mixture; toss to coat.
3In small bowl, mix mayonnaise, milk, honey, mustard, salt and remaining thyme. Cover chicken mixture and mayonnaise mixture separately; refrigerate at least 4 hours but no longer than 24 hours.
4Up to 2 hours before serving, toss chicken mixture and mayonnaise mixture. Cover; refrigerate until serving. Just before serving, stir in 3/4 cup of the walnuts. Sprinkle salad with remaining walnuts.