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.