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1
Cut onions crosswise into 1/2-inch-thick slices and separate slices into rings, reserving small inner rings for another use if desired.
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2
Stir together buttermilk, 2 teaspoons salt, and 1 teaspoon pepper in a large bowl, then gently stir in onion rings.
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3
Let stand, turning occasionally, 10 minutes.
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4
Drain in a colander.
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5
Whisk together flour, remaining 2 teaspoons salt, and remaining teaspoon pepper in a wide shallow bowl.
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6
Dredge onion rings a few at a time in flour, shaking off excess, and arrange in 1 layer on wax-paper-lined trays.
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7
Let onions stand 15 minutes (for batter to set).
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8
Preheat oven to 200F.
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9
Melt enough shortening in a deep 10- to 12-inch heavy skillet to measure about 1 inch and heat over moderately high heat until it registers 360F on thermometer.
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10
Fry onion rings in batches of 4 to 6, without crowding, turning over once or twice, until golden, about 3 minutes per batch.
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11
Transfer as cooked to paper towels to drain, then transfer to 1 or 2 large baking sheets (overlap rings slightly if necessary) and keep warm in oven while frying remaining batches.
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12
(Return oil to 360F between batches.)
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13
Serve immediately.
