• Make-Ahead

Ingredients

1. 1 1/2 pounds Yukon Gold potatoes, peeled and cut into 2-inch chunks
2. Salt
3. 3/4 cup warmed milk
4. Freshly ground pepper
5. 3 ounces mortadella, cut into 1/4-inch dice
6. 5 ounces fresh mozzarella, preferably buffalo, cut into 1/2-inch dice
7. 1 tablespoon unsalted butter, cut into small pieces

Directions

1. Preheat the oven to 425°. Butter a 7-by-11-inch baking dish. In a saucepan, cover the potatoes with water and bring to a boil over moderately high heat. Salt the water and cook the potatoes until tender, about 12 minutes; drain. Shake the potatoes over moderately high heat until very dry. Remove from the heat.
2. Mash the potatoes, stirring in the milk. Season with salt and pepper. Fold in the mortadella and mozzarella, then spread the potatoes in the prepared baking dish. Dot the top with the butter and bake in the upper third of the oven for about 12 minutes, or until bubbling. Turn on the broiler and broil for about 2 minutes, or until crusty and browned on top. Let rest for 5 minutes before serving.

Make Ahead

The sformato can be assembled in the baking dish and refrigerated overnight. Bring to room temperature before baking.