Combine the chicken, apple, celery, and onions in a medium bowl. In a small bowl, combine the mayonnaise, lemon juice, tarragon, parsley, chives, salt and pepper. Pour dressing over the chicken mixture and stir to combine. Refrigerate for at least 1 hour. Just before serving, lightly dress arugula with olive oil. Serve chicken salad on a bed of arugula and top with cashews.