Topping
Cut the potatoes into round slices, transfer these slices to a high pot, add the whole milk, the salt, the oregano and the black pepper, mix it all together very well.
Bring it to a boil over medium heat, cover the pot and let it boil. Open the pot, stir gently with a spoon and cook for another 10 minutes without a lid.
With a fork, check if they have cooked through.
Turn off the heat, add the mozzarella cheese and mix it all together until the cheese fully melts.
Transfer to a glass baking dish and spread it out.
Top it with another 300g of grated mozzarella cheese, or to taste.
Bake in a preheated oven at 200°C (392°F) for 25 minutes.