Heat oil to 370 degrees in a wok or large pot. In a medium bowl, set over an ice bath, stir yolk with chopsticks or a whisk, add water, stirring to combine. Sift flour and cornstarch into egg mixture, stirring to incorporate. Dont worry about the few remaining lumps in the batter. Coat onion rings with batter, letting excess drip back into bowl. Fry onion rings, turning once, in small batches in the oil 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. Makes 4 servings. Comments: When Martha visited Tokyo years ago, Ten Masa, a tempura master, shared with her a few golden rules. When making batter, use ice water, cake flour, cornstarch and egg yolks. Always cook in fresh cottonseed oil; skim frequently for bits of cooked batter; and be sure to change the oil when it becomes discolored. Time is of the essence when making and serving tempura onion rings. The batter should be prepared just before cooking and should be ice cold so that when the batter-coated food hits the hot oil, it seals quickly, and the food is cooked instantly. Since tempura waits for no one, serve it immediately. A small basket or plate lined with a folded piece of special absorbent paper known as shiki-shi is the traditional manner of serving tempura. Accompany tempura onion rings with a dipping sauce of soy, wasabi and grated daikon or follow the decidedly American route of pairing the onion rings with a hamburger and ketchup. Source: "Martha Stewart Living - (www.marthastewart.com)" S(Formatted for MC5): "by Lynn Thomas - Lynn_Thomas@prodigy.net" Per serving: 150 Calories (kcal); 2g Total Fat; (9% calories from fat); 4g Protein; 30g Carbohydrate; 53mg Cholesterol; 6mg Sodium Food Exchanges: 1 1/2 Grain(Starch); 0 Lean Meat; 1 Vegetable; 0 Fruit; 0 Fat; 0 Other Carbohydrates Recipe by: Martha Stewart Converted by MM_Buster v2.0n.