Peel onions; halve lengthwise, through stem. Cut halves crosswise in thin slices. Separate layers and place in bowl. Quarter apples and core them. Cut quarters crosswise into paper-thin slices and toss with onions.
Combine vinegar, sugar, salt, celery seeds, and cayenne, and stir to dissolve. Add buttermilk and stir; add oil gradually, beating with whisk to ble