Step 1

Preheat the oven to 400 degrees F. Butter a 13- by 9- by 2-inch baking dish. Set aside.

Step 2

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.

Step 3

For the peas: In a medium skillet, melt the butter over medium heat. Add the thawed peas and cook until heated through. Add the salt and pepper.

Step 4

Using a large spoon, make a 4-inch well in the center of the potatoes and fill with the cooked peas.

Step 5

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).

Step 6

Drizzle the top with olive oil and bake until the topping is golden brown, about 20 minutes.