Preheat oven to 350℉. Spray an approximate 11"x7" shallow casserole or lasagna pan with non stick spray. If béchamel was chilled, warm over low heat before proceeding. Spread 1/2 cup sauce evenly over the bottom of prepared pan. Top with lasagna sheets. Spread 1/3 of mushroom mixture evenly over lasagna sheet. Ladle 1/2 cup sauce over layer and sprinkle with a 2-3 tbsp of mozzarella cheese. Repeat layer two more times, pasta, 1/3 mushroom mixture, 1/2 cup sauce, sprinkle of cheese. Top with last layer of lasagna noodles and 1/2 cup sauce, and remainder of mozzarella cheese. Cover with foil and bake 45-60 minutes or an instant read thermometer reads 170℉. Remove foil and broil for 2-3 minutes until cheese is melty and golden. Remove from oven and allow to rest for 10-15 minutes before cutting and serving.