Peel and slice onion very thin.

Place onion rings and pieces in a baking dish.

Pour in buttermilk. Allow to soak for at least one hour at room temperature. If soaking for longer, place in the refrigerator.

In a medium bowl, add flour, salt, black pepper and cayenne pepper. Mix to combine. Set aside.

Pour oil into a large pot, Dutch oven or fryer.

Heat oil to 375°F.

When oil is hot and shimmering, grab a handful of onions, allowing excess buttermilk to dip off, and toss into flour mixture. Turn to fully coat all pieces. Tap and shake off excess flour.

Carefully place coated rings into hot oil. Fry for a few minutes until golden. Remove as soon as golden brown to a paper towel-covered plate to drain. Repeat until onions are gone.

Serve immediately as hot as possible.