White Lasagna with Mushrooms and Prosciutto
You can assemble the lasagna, cover it tightly and store it in the refrigerator for up to 1 day, or double-wrap it and put it in the freezer for up to 1 month.
1/2 cup (4 oz./125 g.) unsalted butter, plus more for greasing
1/2 cup (2 1/2 oz./75 g.) unbleached all-purpose flour
1/2 tsp. freshly grated nutmeg
4 cups whole milk
1 cup (8 fl. oz./250 ml.) chicken stock or low-sodium broth
1/2 cup (4 fl. oz./125 ml.) Marsala
2 large eggs, lightly beaten
1 3/4 cups (7 oz./220 g.) shredded fontina cheese
Sea salt and freshly ground white pepper
4 Tbs. (2 fl. 0z./60 ml.) olive oil, plus more if needed
1 small leek, white and tender green parts only, thinly sliced
3/4 lb. (375 g.) white mushrooms, brushed clean and sliced
1/2 lb. (250 g.) thinly sliced prosciutto, chopped
2 Tbs. chopped fresh basil
12 sheets no-boil lasagna, each about 7 by 3 1/2 inches (18 by 9 cm.)
1/2 cup (2 oz./60 g.) freshly grated Parmesan cheese
Preheat the oven to 350 degrees F (180 degrees C). Butter an 8-by-11-inch (20-by-28 cm.) baking dish.
In a large saucepan, melt the butter over medium-low heat. Stir in the flour and cook, whisking constantly, for about 3 minutes. Whisk in the nutmeg. Raise the heat to medium-high and gradually whisk in the milk and stock. Bring the mixture to a boil and continue whisking until thickened and smooth, 10 to 15 minutes. Remove from the heat and let cool, stirring occasionally, until warm. Stir in the Marsala, eggs, and 1 cup (4 oz./125 g.) of the fontina until the cheese melts and the sauce is smooth. Season with salt and white pepper. Set aside.
In a large fry pan, heat 2 tablespoons of the olive oil over medium-high heat. Add the leek and saute just until wilted, about 3 minutes. Transfer to a bowl. Add the remaining 2 tablespoons olive oil to the pan. When the oil is hot, add the mushrooms and saute, stirring, until golden, about 5 minutes. Add the prosciutto and basil and cook, stirring, for 1 minute longer.
Spread about 1 1/2 cups (12 fl. oz./375 ml.) of the cheese sauce evenly over the bottom of the prepared baking dish. Spoon about one-third of the mushroom mixture evenly over the sauce, then arrange 4 of the lasagna sheets over the top. Repeat these layers twice. Spread the remaining sauce over the top layer, then sprinkle the remaining 3/4 cup (3 oz./95 g.) fontina and the Parmesan evenly over the top. Bake until the top is golden and the juices are bubbling, about 45 minutes. Let the lasagna cool for 15 to 30 minutes, cut into squares and serve right away. Serves 6 to 8.