Heat oil in a pan, and add the onions, carrots, and cucumber. Fry for 5 minutes. Pour in the fruit juice, vinegar, Worcestershire sauce, and sugar. Simmer for 10 minutes stirring occasionally. Meanwhile, cook fish in a mod. oven (turning them over half way through) for about 20 minutes. Add the corn flour and water mixture to the sauce, and bring back to a boil, stirring constantly. Simmer for 1- 2 minutes. Serve hot with long grain or basmati rice.