Drain soaked mushrooms and then mince. Reserve soaking liquid but strain it through a cheesecloth lined sieve. First make the cream sauce: Heat 4 tablespoons of the butter in a heavy stainless saucepan. When foaming subsides, add the flour and stir continuously with a wooden spoon. Cook for 2 minutes over low heat. Whisk in the reserved mushroom juices and heavy cream and whisk until smooth. Simmer 5 minutes or until velvety and season to taste with salt pepper and nutmeg. Reserve until later. Now make the filling: In a large skillet heat remaining 3 tablespoons of butter along with the olive oil. Add the fresh mushrooms and garlic and saute for 30 seconds. Add the minced soaked mushrooms, prosciutto and saute a moment. Add the wine and evaporate for a minute or two. Lower the heat and cook, covered until the mushrooms are tender and most of the liquid has been absorbed. Cut the pasta sheets into strips 3 to 4 inches wide and long enough to fit into your gratin or baking dish. Boil a few sheets at a time until 3/4 done. Drain and cool on clean cloths. To assemble: Preheat oven to 375 degrees. Butter a gratin dish. Cover with a layer of lasagne. Top with 1/4 of cream sauce, 1/4 of Parmesan and 1/3 of mushroom mix. Continue with pasta, sauce, cheese, mushrooms for 2 more layers, finishing with a sheet of pasta. Drizzle any liquid left in mushroom pan over the top. Bake for 20 minutes or until golden.