Preheat the oven to 180°C. Place the sliced aubergines in a bowl, sprinkle with salt and wait for 30 minutes for any water to drain off. Place onto kitchen paper to dry.
In a pan, add the olive oil and saute the onion over a moderate heat until softened, add the oregano and the tin of tomatoes simmer for 20 minutes. Add more oil to the pan and saute the potato and aubergine slices to colour.
In a greased gratin dish, build layers of the potato and aubergine slices with the tomato mixture and the sliced tomatoes. Finish with the breadcrumbs and Parmesan and place in the oven to cook for 30 minutes until brown and crisp.