Boil the potatoes in salted water until tender, then peel and cut into slices (large slices cut in half). Cut the tomatoes into small cubes, chop the onions (if you want something to bite later, you can chop roughly).
Sweat the onions in a little olive oil, then add the potatoes and fry them. Then add the tomatoes, mix everything and let it steep for a few more minutes.
Season with salt, pepper, vegetable stock and oregano and season to taste. If you like it spicy, you can also add chilli or paprika powder or something similar.
The whole thing gets a bit mushy if you let the tomatoes stew for a long time, but the potatoes make it wonderfully creamy.