1. In a deep fryer or large saucepan, heat the oil the 350F. Meanwhile, put 1 cup of the cornstarch in a brown paper bag. Add the onion slices and shake well to coat. Transfer the coated onion rings to a large baking sheet. 2. In a large bowl, mix the remaining 1/2 cup cornstarch with the flour and 1 teaspoon salt. Add 1-1/2 cups of cold water all at once into the flour mixture. Stir vigorously with chopsticks or a fork to be pieces of ice and lumps the size of a nickel in the batter. 3. When the oil is hot, add as many coated onion rings to the batter as will fit comfortably in the fryer; you will have to do this in about 5 batches: Using chopsticks or a large fork, life the rings to the hot oil and separate any that cling together. Fry the onion rings until golden brown all over, about 3 minutes. Transfer to paper towels or a paper bag to drain well. Repeat with the remaining onion rings and batter. Sprinkle the onion rings with salt if desired. Serve hot, warm or at room temperature with lemon, ketchup or soy sauce. Serves 4 to 6. Food & Wine, June, 1991.