These delectable, crunchy onion rings are Polo Dobkin's favorite late-night snack. His staff at Dressler devours them too. "We tend to feed a lot of people after hours," he says. To allow room for the light batter on the thick rings to puff as it cooks, use at least 3 inches of oil in the pan.
2 large Spanish onions, cut into 1/2-inch slices and separated into rings
In a very large pot or deep fryer, heat the oil to 365°. In a bowl, whisk the cake flour with the cornstarch, baking soda and 2 teaspoons of kosher salt. Add 2 1/2 cups of the club soda and whisk until smooth; the batter should be thick but pourable. If necessary, add the remaining 1/2 cup of club soda, a few tablespoons at a time.
Working with 6 to 8 onion rings at a time, dip them in the batter until completely coated; as you pull the rings from the batter, scrape some of the excess against the side of the bowl. Carefully add the onion rings to the hot oil and fry until golden and crisp, about 2 minutes. Drain the onion rings on paper towels. Sprinkle the onion rings with salt and serve immediately or keep warm in an oven on low heat while you coat and fry the rest.