4 pounds potatoes, peeled and cut into 1-inch pieces
1 cup whole milk (or skim, if you're watching your cholesterol)
¼ cup butter, melted (Giada suggest ½ cup)
1½ cups grated mozzarella
1 cup freshly grated Parmesan, divided
Salt and freshly ground black pepper
2 tablespoons plain dry bread crumbs
Preheat oven to 400ºF.
Coat a 13x9-inch baking dish with oil.
Bring a large pot of salted water to boil. Cook the potatoes in the boiling water until very tender, about 12 minutes. Drain the potatoes and return them to the same pot. Mash well and mix in the milk and butter. Mix in the mozzarella and ¾ cup parmesan cheese. Season with salt and pepper to taste.
Transfer the potatoes to the prepared baking dish. The dish can be refrigerated at this point for up to six hours. (That's the official time. I don't see why you can't do it overnight.) Stir together the bread crumbs and remaining parmesan cheese. Sprinkle the bread crumb mixture over the mashed potatoes. Bake, uncovered, until the topping is golden brown, about 20 minutes.