• 1 tablespoon butter
  
• 4 pounds russet potatoes, peeled and cooked to mash
  
• 1 cup whole milk
  
• 1/2 cup butter, melted
  
• 1 1/2 cups grated mozzarella
  
• 1 cup grated cheese
  
• Salt and freshly ground black pepper
  
• 2 tablespoons plain dry bread crumbs

Directions

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.

Bake, uncovered, until the topping is golden brown, about 20 to 30 minutes.