For the cod Sicilian style, preheat the oven to 200 degrees with top and bottom heat.
Pat the fish dry, pepper, salt and sprinkle with lemon juice.
Cut the tomato, mozzarella and onion into small cubes and put them in a small bowl.
Add two tablespoons of bread crumbs and mix everything together. Season the mixture with oregano, salt and pepper to taste.
Grease a small baking dish with butter and put the cod fillets in it. Spread the tomato-mozzarella mixture on the surface of the fillets with a small spoon.
If there is some left over, you can put the rest between the fillets. Put the casserole in the oven and cook for 30 minutes.
Arrange the cod in Sicilian style and serve.