Preparation

• Put the onions in a large bowl and add enough buttermilk to cover. Cover the bowl and refrigerate for at least 1 hour and up to 12 hours. In a cake pan or pie plate, combine the flour, cornmeal, and salt.
• Position a rack in the center of the oven and heat the oven to 200°F. Have ready a large rimmed baking sheet fitted with a rack. Line a plate with a few layers of paper towels.
• Pour 1 inch of oil into a 5-quart Dutch oven or other heavy-duty pot and clip a deep-fry thermometer to the side of the pot. Heat over medium heat until the thermometer registers 350°F. Working in batches of 4 or 5, remove the rings from the buttermilk, letting any excess drip off, then dredge in the flour mixture, knocking off the excess.
• Carefully lower the rings into the oil and fry until lightly browned on one side, about 2 minutes. Using tongs, gently turn the rings over and continue to fry, adjusting the heat as necessary to maintain a steady temperature, until golden-brown all over, about 2 minutes. Transfer to the paper towels to drain. Sprinkle generously with kosher salt, then transfer to the rack on the baking sheet and keep warm in the oven.
• Continue to fry the onions in batches, letting the oil return to 350°F between batches and replacing the paper towels on the plate as needed. Serve with ketchup, if you like.