How to Make It
Sift the flour into a large bowl. Whisk in the water, then, while whisking, add the seltzer, club soda or sparkling water, stopping when the batter is the consistency of very thick heavy cream.
In a large, deep saucepan, heat the vegetable oil to 350°. Dip 8 to 10 of the onion rings in the batter to coat. Using chopsticks, lift out the onion rings, allowing the excess batter to drip back into the bowl, and add the rings to the hot oil. Fry, turning once, until the rings are deep golden and crisp, about 4 minutes. Drain on paper towels, sprinkle with kosher salt and serve. Repeat to make the remaining onion rings.