Preparation Separate slices into rings, reserving small inner rings for another use if desired. Stir together flour, baking powder, and 1 teaspoon salt in a bowl. Whisk together egg and milk in another bowl. Spread bread crumbs out on a plate. Coating rings in batches of 4, dredge rings in flour mixture, shaking off excess, then dip in egg mixture, letting excess drip off, and dredge in crumbs, shaking off excess. Transfer to sheets of wax paper. Heat 1 inch oil in a 5-quart pot over medium-high heat until it registers 360°F on thermometer. Fry rings in batches of 4 to 6, without crowding, turning over once or twice, until golden brown, about 3 minutes per batch. Transfer to paper towels to drain. Cooks' note:Onion rings can be made 4 days ahead. Transfer, once cooled, to wax-paper-lined baking sheets and freeze until firm, about 10 minutes, then transfer to 2 large sealable bags and freeze. Reheat on 2 large baking sheets in a 425°F oven with racks in upper and lower thirds, switching position of sheets halfway through, about 8 minutes total. Transfer to racks to crisp 1 minute.
Preparation Separate slices into rings, reserving small inner rings for another use if desired. Stir together flour, baking powder, and 1 teaspoon salt in a bowl. Whisk together egg and milk in another bowl. Spread bread crumbs out on a plate. Coating rings in batches of 4, dredge rings in flour mixture, shaking off excess, then dip in egg mixture, letting excess drip off, and dredge in crumbs, shaking off excess. Transfer to sheets of wax paper. Heat 1 inch oil in a 5-quart pot over medium-high heat until it registers 360°F on thermometer. Fry rings in batches of 4 to 6, without crowding, turning over once or twice, until golden brown, about 3 minutes per batch. Transfer to paper towels to drain. Cooks' note:Onion rings can be made 4 days ahead. Transfer, once cooled, to wax-paper-lined baking sheets and freeze until firm, about 10 minutes, then transfer to 2 large sealable bags and freeze. Reheat on 2 large baking sheets in a 425°F oven with racks in upper and lower thirds, switching position of sheets halfway through, about 8 minutes total. Transfer to racks to crisp 1 minute.