Put onion rings in a large bowl and add enough buttermilk to cover. Cover bowl and refrigerate for at least 1 hour and up to 12 hours. In a pie pan combine flour, cornmeal and salt.
Position a rack in the center of the oven and heat the oven to 200F. Line a plate with paper towels.
Pour 1 inch oil in a 5Qt Dutch oven and clip thermometer to the side of the pot. Heat over medium heat until temp reads 350F. Working in batches remove onions from buttermilk, letting excess drip off, then dredge in the flour mixture, knocking off the excess.
Add onion rings to oil in batches, cook for about 4 minutes flipping half way through. Remove onion rings when golden. Make sure temp stays at 350F. Sprinkle with salt and keep warm in oven.
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