Heat 50g (approximately 1 3/4 ounces) of butter, add flour and cook. Stir the milk in with a whisk and simmer for 3 minutes, stirring constantly. Stir in the crème fraîche and season with salt, pepper and lemon juice.
Rinse and slice the porcini. Cut the Prosciutto into cubes. Heat the olive oil in a large skillet and sauté the porcini. Add the Prosciutto. Rinse the parsley, shake dry and chop coarsely. Mix into the mushrooms. Season with salt and pepper.
In a greased baking dish, place a layer of the lasagna noodles and top with some of the sauce. Layer the mushroom mixture on top and sprinkle with some Parmesan. Repeat. Cover the top layer with lasagna noodles, some sauce and cover with a few mushrooms and sprinkle with the remaining Parmesan.
Top the lasagna with the remaining butter cut into small pieces. Bake in a preheated oven at 200-220° (approximately 400-425°F) for about 30 minutes.