Take a pan, fill 3/4 full with lightly salted water, set it on the hob over high heat, and when the water begins to boil, boil the pasta for about 1 minute per side. Drain, and let them dry on a clean cloth without overlapping otherwise they will stick.
Clean the mushrooms well (if using frozen transfer them directly into the pan), slice them and let them cook for about 5 minutes in a pan with olive oil, parsley, a clove of garlic and a pinch of salt, stirring well from time to time.
Take 400ml of the bechamel sauce and melt the 100g of the mozzarella in it. Stir the garlic and mushrooms into the sauce. Stir in the cream until the mixture is creamy looking.
Grease a baking dish of about 25x35 cm and arrange in layers, alternating lasagne sheets with white sauce mixture. Finish with a final layer of lasagne, then cover with the remaining béchamel sauce, dot with flakes of butter here and there and sprinkle over plenty of Parmesan cheese.
Bake for about 1/2 hour. Let your lasagna rest for at least 10 minutes before serving.