1/2 cup unsalted butter, plus more for greasing
Sea salt and freshly ground white pepper
1/2 cup unbleached all-purpose flour
4 tablespoons olive oil, plus more if needed
1/2 teaspoon freshly grated nutmeg
1 small leek, white and tender green parts only, thinly sliced
4 cups whole milk
3/4 pound white mushrooms, brushed clean and sliced
1 cup chicken stock or low-sodium broth
1/2 pound thinly sliced prosciutto, chopped
1/2 cup Marsala
2 tablespoons chopped fresh basil
2 cloves large eggs, lightly beaten
12 sheets no-boil lasagna, each about 7 by 31/2 inches
1 3/4 cup shredded fontina cheese
1/2 cup freshly grated Parmesan cheese
Preheat the oven to 350°F (180°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, 10–15 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 frying pan, heat 2 tablespoons of the olive oil over medium-high heat. Add the leek and sauté 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 sauté, stirring, until golden, about 5 minutes. Add the prosciutto and basil and cook, stirring, for 1 minute longer.
Spread about 11/2 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.
Save time: You can use dried lasagna noodles in this recipe. They cut down on the time and the space you need to prepare the dish, , and you won’t be able to tell the difference in taste or texture.
Make ahead: 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.