In a large pan pour the milk and heavy cream and bring to simmer, then add the potatoes previously peeled and sliced. Add the salt, pepper, ground nutmeg and cook for about 5 minutes with low heat.
Rub the butter all over a casserole or baking dish, then pour half of the potatoes along with the milk and cream. Tear apart pieces of mozzarella cheese with your hands and sprinkle all over the potatoes, then cover with another layer of potatoes and cream.
As last layer, cover the surface with grated Parmesan cheese for an extra crispy gratin.
Bake in the oven for about 40 minutes at 180°C (375°F) or until golden on top.
Serve piping hot!
If you want to make it even more Italian, try adding a couple of tablespoons of Gorgonzola cheese sprinkled together with the Mozzarella.