Toss the cabbage with 1 teaspoon salt and 1/2 teaspoon sugar in a colander set over a bowl. Let stand for 30 minutes. Discard any liquid that collects in the bowl.
Combine the shallots and cider vinegar in a large bowl. Let stand for 5 minutes. Whisk in the buttermilk, mayonnaise, mustard, remaining 1/4 teaspoon sugar and cayenne.
Gently squeeze any excess moisture from the cabbage. Add the cabbage to the dressing bowl, along with the parsley, walnuts, celery and apples. Toss and taste for seasoning. Add more salt and pepper if desired.