2) For the white sauce, bring the milk to a simmer in a saucepan. Set aside. Melt 110g of the butter in a large saucepan. Add the flour and cook for 1 minute over low heat, stirring constantly with a wooden spoon. Pour the hot milk into the butter-flour mixture all at once, stirring all the time.
3) Add 1 tbsp salt, the pepper and nutmeg, and cook over a medium-low heat, stirring first with the wooden spoon and then with a whisk, for 3 to 5 minutes, until thick. Set aside off the heat.
4) Remove the mushroom stems and discard; slice the caps 1cm thick. Heat 2 tbsp of oil and the remaining butter in a large frying pan. When the butter melts, add half the mushrooms, sprinkle with salt, and cook over medium heat for about 5 minutes, until the mushrooms are tender and release their juices. If they become too dry, add a little more oil. Toss occasionally so the mushrooms cook evenly. Repeat with the remaining mushrooms; set all the mushrooms aside.
5) To assemble the lasagne, spread some of the white sauce in the bottom of a 16x24x4cm deep baking dish. Arrange a layer of lasagne sheets on top, then more sauce, then 1/3 of the mushrooms and 25g of the grated Parmesan.
6) Repeat 2 more times, layering pasta sheets, sauce, mushrooms, and Parmesan. Top with a final layer of pasta sheets and sauce and sprinkle with the remaining Parmesan.
7) Bake the lasagne for 45 minutes in the oven, or until the top is browned and the sauce is bubbly and hot. Allow to sit at room temperature for 15 minutes and serve hot.