In a splendid bowl, creep the flour with the cornstarch, baking powder and the two teaspoons of kosher salt. Creep in 3 cups of the club soda till the batter is refined. Add extra club soda, about 1 tablespoon at a time, till the combination is the consistency of thin pancake batter. Let stand for 10 minutes.
Within the period in-between, in a splendid saucepan, heat the vegetable oil to 380°. Dip 8 onion rings within the batter, letting the excess drip back into the bowl. Fry the onion rings within the hot oil over high heat, turning every so frequently, till golden and crispy, about 3 minutes. Transfer the onion rings to a paper towel-lined baking sheet and sprinkle lightly with salt. Repeat with the very finest onion rings and batter and support hot or heat.