How to Make It
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.