Slice the onion very thin. Place the onion in a 13x9 inch baking dish. Pour the buttermilk over the onions and let them soak for at least one hour. Combine the flour, salt,( cayenne pepper if using) and pepper in a large bowl. Whisk to combine. Place the oil in a skillet and heat to 375 degrees. Working in batches...take a handful of onions and dredge them into the flour mixture, shake off any excess flour and place in the skillet with the hot oil. Fry until onions are golden brown (it only takes a couple on minutes) then place on a paper towel lined plate. Repeat until all the onions are fried.
