To assemble lasagna, pour about 1 1/2 cup of marinara sauce into the bottom of the prepared baking dish and top the sauce with 4 lasagna noodles, overlapping if necessary. On the noodles, layer 1/3 of the ricotta mixture, followed by 1/3 of the cooked sausage, 1/3 cup of mushrooms, 1/4 of the bechamel sauce, 1 cup of the mozzarella cheese, 1/8 cup of the Parmesan cheese and 1 more cup of marinara sauce. Repeat the layers twice more, ending with a final layer of noodles topped with a layer of marinara sauce, bechamel sauce, the remaining mozzarella cheese, and remaining Parmesan cheese. Cover the dish with foil.