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1
Place onion rounds, 2 cups beer, vinegar, 1/2 teaspoon salt, and 1/2 teaspoon pepper in zipper-lock bag. Refrigerate 30 minutes or up to 2 hours.
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2
Heat the oil in a large Dutch oven over medium-high heat to 350 degrees.
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3
While oil is heating, combine flour, cornstarch, baking powder, 1/2 teaspoon salt, and 1/4 teaspoon pepper in large bowl.
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4
Slowly whisk in 3/4 cup beer until just combined (some lumps will remain).
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5
Whisk in remaining beer as needed, 1 tablespoon at a time, until batter falls from whisk in steady stream and faint trail across surface of batter.
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6
Adjust oven rack to middle position and heat oven to 200 degrees.
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7
Remove onions from refrigerator and pour off liquid.
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8
Pat onion rounds dry with paper towels and separate into rings.
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9
Transfer 1/3 portion of rings to batter.
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10
One at a time, carefully transfer battered rings to oil.
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11
Fry until rings are golden brown and crisp, about 5 minutes, flipping halfway through frying.
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12
Drain rings on paper towel-lined baking sheet, season with salt and pepper, and transfer to oven.
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13
Return oil to 350 degrees and repeat with remaining onion rings and batter.
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14
Serve.
