Boil the potatoes and put them through the potato press or mash them into a puree.
Finely grate the grana or parmesan, cut the mozzarella into small pieces and add to the potatoes with the milk, olive oil, parsley, basil and nutmeg. Mix everything together well and season with salt and pepper.
Oil a baking dish, pour in the mashed potatoes and let it gratinate at 200 degrees for about 10 minutes.