The delicate texture of the lemon sole is well complemented by this very light batter.
About 1½poundslemon sole fillets
Vegetable oil for deep-frying
1 large egg
3tablespoonsrice or white wine vinegar
2teaspoonsthin soy sauce
Prepare the batter. Sift the flour into a mixing bowl and stir in the egg. Add the water gradually, stirring to blend into a smooth batter, the consistency of cream. Add the salt. Leave to stand for 20-30 minutes, then blend in the oil to make it smoother.
Prepare the sauce. Dissolve the cornstarch in about 2tablespoons of the water; add the vinegar, sugar, salt, soy sauce, Worcestershire sauce, and ketchup. Stir in the remaining water. Pour into a saucepan (or a second wok) and cook over medium heat until the sauce thickens, stirring to ensure that it does not become lumpy. Remove from the heat and blend in the oil. Keep nearby.
Pat dry the lemon sole fillets. Halve lengthwise, then cut each half crossways into 2-3 pieces.
Half-fill the wok with oil and heat over a high heat until it reaches a temperature of 375° F, or until a cube of stale bread browns in about 50 seconds. In the meantime, add half of the fish to the batter to coat. Using a pair of bamboo chopsticks or tongs, lift the fish, piece by piece, and put into the oil to deep-fry for about 1 minute, or until pale golden. (When the last piece goes into the oil, the first will be almost ready to come out.) Remove onto paper towels to drain. Coat and deep-fry the remaining fish as before.
Reheat the oil to the same temperature. Add all the fish to deep-fry a second time for about 30 seconds, crisping the batter to a turn. Remove with a large hand strainer or perforated disc onto paper towels, then transfer to a serving plate.
Reheat the sauce, pour over the fish, and serve immediately. (The sauce can also be served in a bowl for individuals to use as a dip.)