Thinly slice the onion using a mandolin slicer or very sharp knife. Separate onion rings and place into a shallow dish.
Cover the onions with buttermilk, tossing until all covered. Set aside at room temperture, tossing every 10-15 minutes to ensure they are all marinating evenly.
In a medium mixing bowl, combine the flour, salt, ground black pepper and celery salt.
Fill a large, heavy bottom pan with 2 inches of vegetable oil and heat to 350°F.
Working in batches, use a slotted spoon or your fingers to remove onions from buttermilk, tapping to remove any excess liquid. Drop into flour mixture, turn to coat and shaking off excess flour. Then carefully place in hot oil.
Fry for 2-3 minutes, or until lightly brown. Remove to a wire rack over a baking sheets.
Continue with the remaining onions.
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