In a small mixing bowl, combine mayonnaise, buttermilk, lemon juice, dill, parsley, garlic powder, salt and pepper. Stir until well combined. Set aside. In a large mixing bowl, toss together chicken, grapes, cashews, celery, green onion and Parmesan. Pour dressing over chicken mixture and toss mixture until well coated. Serve on a bed of lettuce (if desired) on sliced bread, croissants, rolls, or French bread.