Italian Sausage and Mushroom Lasagna with B?chamel Sauce

Step 1
Preheat the oven to 375 degrees F (190 degrees C). Grease a 9×13-inch baking dish with cooking spray.

Step 2
Place sausage in a skillet over medium heat and cook until browned and crumbly, about 10 minutes. Drain and set aside.

Step 3
To make b?chamel sauce, melt butter in a saucepan over medium heat and whisk in flour. Cook until light and foamy, about 2 minutes, whisking constantly; whisk in milk, a little at a time. Whisk in salt and nutmeg. Bring sauce to a bare simmer, whisking constantly until thickened, about 10 minutes. Remove sauce from heat and set aside.

Step 4
Beat egg in a medium bowl; stir in ricotta cheese and parsley until thoroughly combined. Set aside.

Step 5
To assemble lasagna, pour about 1 1/2 cups marinara sauce into the bottom of the prepared baking dish. Top sauce with 4 lasagna noodles, overlapping if necessary. Layer 1/3 of the ricotta mixture, followed by 1/3 of the sausage, 1/3 cup mushrooms, 1/4 of the b?chamel sauce, 1 cup mozzarella cheese, 1/8 cup Parmesan cheese, and 1 more cup marinara sauce over noodles. Repeat the layers twice more, ending with a final layer of noodles topped with remaining marinara sauce, b?chamel sauce, mozzarella, and Parmesan cheese. Cover the dish with foil.

Step 6
Bake in the preheated oven until bubbly and noodles are tender, about 1 hour. Remove the foil and bake until the top cheese layer is browned, about 5 more minutes. Allow lasagna to stand for 10 minutes before cutting.