In a large bowl, combine the cooked chicken, red grapes, celery, walnuts, and green onions.
In a separate small bowl, whisk together the mayonnaise, Dijon mustard, apple cider vinegar, honey, salt, and pepper until smooth.
Pour the dressing over the chicken mixture and gently toss until everything is well coated.
Taste and adjust seasoning if necessary.
Cover the bowl and refrigerate for at least 30 minutes to allow the flavors to meld.
Serve the chicken salad on a bed of lettuce leaves or as a sandwich filling.