Preheat the oven to 200 ° C top / bottom heat (180 ° C convection).
Peel the potatoes and cut or slice them into approx. 5 mm thick slices. Peel the onions and cut into thin slices. Cut the tomatoes into eighths. Chop the garlic.
Put the olive oil in an ovenproof dish, then add the potatoes, tomatoes, onions, possibly garlic and water and mix.
Bake everything for about 1 hour until the potatoes are tender and stir every 20 minutes. After about 40 minutes add the cheese and oregano. After the baking time, season to taste with salt and pepper and let rest for approx. 10 minutes.