method
1. Preheat the oven to 325°F (170°C) mark 3.
2. Peel the potatoes and cut into slices about 3/8 in (1 cm) thick; as you cut the slices, put them in a large bowl of cold water to prevent them turning brown.
3. Peel the onions and slice very thinly.
4. Drain the tomatoes, reserving the juice. Cut the tomatoes open and spoon all their seeds into the sieve; drain off any more juice they release and keep it. Discard the seeds. Chop the flesh coarsely.
5. Grease a wide, shallow ovenproof dish with oil.
6. Drain the potatoes and transfer to the oven dish together with the onions and tomatoes. Sprinkle with the reserved tomato juice. Season with salt, freshly ground pepper and sprinkle with the oregano and the grated cheese.
7. Finally, moisten with about 9 fl oz (250 ml) boiling water and sprinkle with 2 tbsp olive oil.
8. Bake for 1 1/4 - 1 1/2 hours or until the potatoes are very tender and the top is crisp and golden. Allow to stand for 10 minutes before serving.
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