Slice the onion rings as thickly as you prefer.
Pour buttermilk into a bowl, add the raw onion rings, and marinate for at least 30 minutes.
In a large bowl, whisk together flour, cornstarch, and cayenne pepper.
Dredge the onion rings in the flour mixture, shaking off any excess.
Heat the oil in a deep skillet over medium-high heat.
Carefully place the onion rings in the hot oil, being sure not to overcrowd the pan.
Fry for 2-3 minutes per side, or until golden brown and crispy.
Use a slotted spoon to transfer the cooked onion rings to a cooling rack with a layer of paper towel underneath, to drain any excess oil.
Repeat with the remaining onion rings, adding more oil to the pan as needed.
Season with salt and serve hot with your favorite dipping sauce.