Separate onion slices into rings, and place in a 9-inch square baking dish. Pour buttermilk over onion rings. Cover and chill 2 to 24 hours.
Pour oil to depth of 2 inches into a Dutch oven; heat to 360°.
Stir together flour and next 3 ingredients in a shallow dish. Dredge onion rings in flour mixture, and place on a baking sheet. Discard buttermilk.
Fry onion rings, in batches, 2 minutes or until golden brown, turning once. Drain on paper towels.
Serve onion rings immediately, or keep warm in a 200° oven until ready to serve.
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