3 tablespoons flour
2 cups milk
Scrapings of nutmeg
1 cup freshly grated Parmigiano-Reggiano, divided, plus more for the table
3/4 pound thinly sliced unsmoked boiled ham (prosciutto cotto), cut into julienne strips
1. Heat the oven to 400 degrees and place a rack at the uppermost position. Cover the dried mushrooms with 3 cups of warm water and set aside to soften at least 15 minutes. Remove the mushrooms from the water (reserving the water), squeeze them dry and chop them finely. Strain the soaking water through a coffee filter to remove any dust.
2. Over medium heat, heat the oil, 3 tablespoons butter and the chopped onion in the largest sauté pan you have, ideally one big enough to hold all the sliced mushrooms without crowding. Cook until the onion turns translucent, about 3 minutes.
3. Add the porcini mushrooms, the soaking water, chopped tomatoes and parsley, and continue cooking until the water has evaporated and the mixture is nearly dry, about 10 minutes.
4. Add the sliced fresh mushrooms, 1 teaspoon salt and one-fourth teaspoon pepper, or to taste, and increase the heat to high. Cook, stirring frequently, until all the moisture released by the mushrooms has evaporated, 10 to 20 minutes, increasing the heat if needed for the liquid to evaporate more quickly. Taste and correct for salt and pepper. Remove from the heat.
5. Make a white sauce: Heat one-fourth cup butter in a saucepan over medium-low heat. When it has melted, whisk in the flour to make a smooth paste. Whisk in the milk about one-half cup at a time, cooking until each addition has thickened. Season with one-half teaspoon salt and a pinch of nutmeg, or to taste, and stir in two-thirds cup of the grated Parmigiano-Reggiano. Remove from heat and hold in a warm place. (A skin will probably form on top of the sauce. Just stir it back in. Any small clumps will melt as the lasagna bakes.)
6. Generously butter a 12-by-9-inch baking dish and lightly coat the bottom with the white sauce. Line the bottom of the pan with a single layer of pasta strips, cutting them to fit the pan, edge to edge, allowing no more than one-fourth inch overlapping.
7. Stir all but about one-half cup of the remaining white sauce into the mushroom mixture. Spread a thin layer of the mushroom mixture on top of the pasta, then scatter a handful of ham strips over the top. Cover with another pasta layer and more mushrooms and ham, repeating until you've used all the mushrooms and ham, finishing with a layer of pasta. Ideally, you'll have at least 6 layers of pasta. Spoon the reserved one-half cup of white sauce evenly over the top layer of pasta, and sprinkle over the remaining one-third cup Parmigiano-Reggiano. (The recipe can be prepared to this point and refrigerated tightly covered.)
8. If baking the lasagna immediately after assembling, place on the top rack and bake until a light golden crust forms on top, about 10 to 15 minutes. If the crust is slow to develop, turn up the oven to 450 or 475 degrees, but do not bake longer than 15 minutes altogether. If the lasagna was refrigerated, place on a rack in the center of the oven (for more even heating) and bake until it is warmed through and beginning to form a light golden crust, 25 to 35 minutes; place on the top rack after warming to form the crust more quickly if needed.
9. Remove from the oven and set aside to settle for about 10 minutes. Then serve with grated Parmesan on the side.
Protein 13 grams
Carbohydrates 22 grams
Fiber 2 grams
Fat 16 grams
Saturated fat 7 grams
Cholesterol 87 mg
Sugar 4 grams
Sodium 755 mg
NOTE: Adapted from Marcella Hazan's "Essentials of Classic Italian Cooking."