Heat the butter separately and fry the flour. Stir constantly, add the cream at room temperature and then pour in the milk until you get the desired consistency.
Stir the béchamel sauce continuously and finally, season it with spices. Mix the fried mushrooms into the sauce. Prepare the lasagna according to the package's directions. At the bottom of a lightly buttered rectangular tray, spread a little of the sauce and start arranging a layer of lasagna sheets, a layer of béchamel mushroom sauce.
At the very top, place lasagna sheets, pour in a trickle of oil and sprinkle with grated cheese. Bake the mushroom lasagna in a pre-heated 392°F (200 °C) oven, until browned.