Cook the potatoes in boiling salted water until almost done. Cut them into thick slices and coat them in olive oil. Arrange them around the edge of a greased ovenproof dish and place the fish in the center.
Heat the olive oil in a frying pan, add the onion and sauté gently for about 5 minutes until soft. Add the garlic and oregano and cook for 2—3 minutes. Add the tomatoes, wine or stock and salt and pepper to taste and simmer for a few minutes.
Spoon half of the tomato mixture over the potatoes and fish in the ovenproof dish. Mix the remaining sauce in a small bowl with the breadcrumbs and cheese and then pour over the fish.
Bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 15 minutes or until the fish is cooked. Sprinkle with the chopped parsley and serve immediately.