This recipe appears in our cookbook "Martha Stewart's Vegetables" (Clarkson Potter). Thinly layered, slightly spicy yellow onions are used to make the light, crisp rings at Balthazar in New York City. (This recipe is adapted from the restaurant's cookbook, and it is Martha's hands-down favorite.) Beer flavors the batter and adds body, thanks to the bubbles. Baking powder, a leavening agent, helps to enlarge these bubbles as the batter fries.
This is essentially the recipe I have been using for decades--since I rarely measure for this recipe it's hard to declare my recipe identical, and I've been doing it for so long, I can't even remember if Martha was my original source. I like using flour and not cornstarch because it browns better. The seasonings are perfect, as is beer for the liquid. For folks who don't cook with alcohol--ignoring the fact that the alcohol all cooks out--club soda is a good substitute. This is also the batter I use for tempura. Make sure the onion rings are dry before dipping in batter so the batter sticks.