- 1 tablespoon unsalted butter, at room temperature, for greasing the baking dish
- 1 tablespoon unsalted butter, at room temperature
- 2 teaspoons kosher salt, plus extra for seasoning
- 4 pounds russet potatoes, peeled, cut into 1-inch pieces
- 1 1/2 cups grated mozzarella
- 1 cup whole milk, at room temperature
- 3/4 cup freshly grated Parmesan
- 1/2 cup (1 stick) unsalted butter, melted
- Freshly ground black pepper
- 2 tablespoons unsalted butter, at room temperature
- 2 cups frozen peas, thawed
- 1/2 teaspoon kosher salt
- 1/4 teaspoon freshly ground black pepper
- 1/2 cup freshly grated Parmesan
- 2 tablespoons plain dry breadcrumbs
- Extra-virgin olive oil, for drizzling
Preheat the oven to 400 degrees F. Butter a 13- by 9- by 2-inch baking dish. Set aside.
For the potatoes: In a large saucepan, combine the butter, salt and potatoes. Add enough cold water to cover the potatoes by 1 inch. Bring to a boil over medium-high heat. Boil until the potatoes are very tender, about 15 minutes. Drain the potatoes and return them to the same pot. Using a potato masher, mash the potatoes until smooth. Mix in the mozzarella, milk, Parmesan and butter. Season with salt and pepper. Transfer the potatoes to the prepared baking dish.
Using a large spoon, make a 4-inch well in the center of the potatoes and fill with the cooked peas.
For the topping: Stir the cheese and breadcrumbs in a small bowl to blend. Sprinkle the breadcrumb mixture over the mashed potatoes (can be prepared up to this point 6 hours ahead of time; cover and chill).
Drizzle the top with olive oil and bake until the topping is golden brown, about 20 minutes.