Peel the onions. Using a mandoline or cut by hand into very thin slices. Cut the slices in half and separate. Put the onion rings in a bowl and pour the buttermilk over them. Either refrigerate until ready to fry or continue on with the recipe. Add the cayenne pepper to the flour. When ready to fry, take a handful of onions out of the buttermilk and put in the flour and shake around until coated When the oil is to 350-375° take a handful of the onion rings, shake a little to remove excess flour and drop in oil. Fry until nice and brown. Remove from oil and place either on paper towels to train or a cookie cooling rack set over a cookie sheet.