Preheat the oven to 425F. Make a roast in a regular form by stacking the fillets on top of each other. Tie with string in a couple of places so the roast won╒t break apart during cooking. Sprinkle with salt and pepper and place in an ovenproof serving dish. Surround the cod with alternating pieces of tomato and mushroom. Sprinkle with oil and white wine and bake, basting from time to time. 10 minutes before the end of cooking, sprinkle with the chopped onion and parsley and remove the string. Serve accompanied by lemon slices and boiled potatoes. You can use frozen cod in the same way, without adding the wine. The frozen block will produce some water and will take at least 10 minutes more cooking time.
