Preheat the oven to 200 C, 180 Fan, Gas 6. Slice the potatoes and onions thinly. Slice the tomatoes thinly, reserving the juices.
Use a roasting tin or ovenproof dish at least 4 cm deep and rub it with oil. Make a layer of potatoes, then onions, then tomatoes and their juices. Dribble with a little oil then sprinkle with cheese and oregano.
Repeat until you have used all the ingredients, with potatoes as the last layer if possible.
Pour over the stock, then finish with a thick layer of breadcrumbs mixed with cheese. Drizzle with more olive oil.
Bake for an hour or so, then remove from the oven and leave to settle a bit before serving.