Sauté onion in butter. Stir in tomatoes, salt, pepper, sugar and Worcestershire sauce. Simmer for 5 minutes.
Place each piece of fish on a fairly large square of aluminium foil. Sprinkle with lemon juice and seasonings.
Pour a little of the tomato sauce over each one, sprinkle with grated cheese. Wrap up loosely.
Bake for about 20 minutes.
Notes
For an easier meal, you can sprinkle the fish only with butter, lemon juice and mixed herbs. Cook in the same way. The fish tastes delicious and moist.