Heat half the oil in a saucepan. Add the onion and cook for 4mins, or until softened. Stir in the tomatoes, oregano, basil, seasoning and mix well.
Bring mixture to the boil. Reduce heat and simmer for 5mins.
Stir in mushrooms and courgettes. Cook, stirring occasionally for 10mins. Preheat grill - if using fresh cod (frozen cod see tip).
Meanwhile, season flour with salt and pepper, toss fish in flour and coat. Heat remaining oil in large frying pan. Fry fish for 5mins on each side, until golden.
Add the mixture to the bottom of the dish, place the fish on top, cover with the cheeses and grill until topping is golden and serve.