In a bowl, whisk together the mayonnaise and vinegar. Slowly add 1/3 cup buttermilk, whisking to thicken. Add the blue cheese, onion, salt, pepper, and cayenne, and whisk well to combine, adding more buttermilk, as desired. Set aside.
In a large bowl, combine the apples, celery, grapes, and walnuts. Add the dressing, to taste, and toss well.
Arrange the lettuce leaves on 6 salad plates. Top with the tossed salad and serve.
Yield: 6 servings