Soak onion rings in a bowl of ice water for 1 hour; drain.

Combine flour and seasonings in a large plastic bag; pour buttermilk into a shallow dish. Dip onion rings in the buttermilk, then transfer a few at a time to the flour using a skewer for easy maneuvering. Dip rings a second time into the buttermilk and flour using the skewers. Set the coated, skewered rings over a bowl while coating the remaining onions.

Heat 3" oil in a pot over medium-high to 365°. Fry half the onions in the oil until golden, 3–4 minutes. Remove from the oil using skewers, then rest over a bowl to drain. Fry remaining onions in the same way.

Soak the onion slices in cold water to dilute the sulfur content, turning them sweet and mild.	For a crust that stays crisp and has character, double dip the rings in buttermilk and seasoned flour.