Peel the potatoes and cut them into uniform slices, not too thin. Put to boil until semi-cooked. In a greased rectangular pyrex tray, start to stack: potato, tomato, and mozarella cut into uniform slices, by overlapping them slightly.
Arrange in columns along the length of the tray. Sprinkle it nicely with salt, black pepper, white pepper and oregano. Distribute blocks of butter and pour over with the beaten in advance eggs and yoghurt, mixed with grated cheese.
The mozzarella gratin is baked in the lower part of the oven on high heat for about 15 minutes.