In a wok or large pot, heat oil until a deep-frying thermometer registers 370 degrees F. In a medium bowl set over an ice bath, stir yolk with chopsticks or a whisk; add ice water, stirring to combine. Sift flour and cornstarch into egg mixture, stirring to incorporate. Don't worry about a few remaining lumps in the batter.
Coat onion rings with batter, letting excess drip back into bowl. Fry onion rings in the hot oil in small batches, turning once, until just beginning to color. Drain on paper towel. Repeat process, skimming oil of tempura batter bits as necessary until all onion rings have been fried. If oil becomes dark, begin again with fresh oil. Serve immediately.
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