What you need:
1 tablespoon butter
4 pounds russet potatoes, peeled, cut into 1-inch pieces
1 cup whole milk
1/2 cup (1 stick) butter, melted
1 1/2 cups grated mozzarella
1 cup freshly grated Parmesan
Salt and freshly ground black pepper
2 tablespoons plain dry bread crumbs
What to do:
Preheat the oven to 400 degrees F. Coat a 13 by 9 by 2-inch baking dish with 1 tablespoon of butter and set aside.
Cook the potatoes in a large pot of boiling salted water until they are very tender, about 15 minutes. Drain; return the potatoes to the same pot and mash well. Mix in the milk and melted butter. Mix in the mozzarella and 3/4 cup of the Parmesan. Season, to taste, with salt and pepper. Transfer the potatoes to the prepared baking dish. Stir the bread crumbs and remaining 1/4 cup of Parmesan in a small bowl to blend. Sprinkle the bread crumb mixture over the mashed potatoes. Recipe can be prepared up to this point 6 hours ahead of time; cover and chill.