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1
Preheat the oven to 350u00b0F. Butter an 8-by-11-inch baking dish.
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2
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.
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3
In a large frying 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.
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4
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.
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5
Let the lasagna cool for 15 to 30 minutes, cut into squares, and serve right away.
