Sara Kate Gillingham-Ryan's White Lasagna with Mushrooms and Prosciutto

Preheat the oven to 350°F. Butter an 8-by-11-inch 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 1 1/2 cups 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 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.