- Preheat the oven to 400 degrees F. Coat a 13 by 9 by 2-inch baking dish with nonstick spray and set aside.
- Cook the potatoes in a large pot of boiling salted water until they are very tender, about 15 minutes. Meanwhile, in a skillet, melt 3 tablespoons of butter. Add thawed peas and sauté until heated through. Season with salt and pepper. Turn off the heat.
- Drain the potatoes and return the potatoes to the same pot and mash well. Mix in the milk and 1/2 cup 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. In the middle of the potatoes, make a 4 inch well and fill with the pea sauté. Combine 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 minutes.