Creating your Mushroom and Spinach Lasagna is simple if you follow these thoughtful steps:

1. Preheat Oven: Preheat your oven to 375°F (190°C).

2. Cook Lasagna Noodles: In a large pot of salted boiling water, cook the lasagna noodles according to package instructions until al dente. Drain and set aside.

3. Sauté Vegetables: In a large skillet, heat olive oil over medium heat. Add onion and sauté until translucent. Then, add garlic and mushrooms, cooking until mushrooms are tender. Add spinach and continue cooking until wilted. Season with salt, pepper, and Italian seasoning.

4. Prepare Béchamel Sauce: In a saucepan, melt 3 tablespoons of butter over medium heat. Whisk in the flour and cook for 1 minute. Gradually add milk, whisking constantly until the sauce thickens. Season lightly with salt and pepper.

5. Layer the Lasagna: In a 9×13 inch baking dish, spread a thin layer of marinara sauce on the bottom. Place 3 lasagna noodles over the sauce. Spread 1/3 of the ricotta cheese over the noodles, followed by 1/3 of the vegetable mixture. Pour 1/3 of the béchamel sauce over the vegetables, then sprinkle 1/3 of the mozzarella cheese. Repeat these layers two more times, finishing with a layer of noodles, marinara sauce, béchamel, and mozzarella cheese on top.

6. Bake: Cover the dish with aluminum foil and bake in the preheated oven for 30 minutes. Remove the foil and bake for an additional 15-20 minutes or until the top is golden and bubbly.

7. Cool and Slice: Once baked, remove from the oven and let it cool for 15 minutes before slicing. This cooling period helps in cutting neat portions.

8. Garnish and Serve: Optionally, garnish with fresh basil before serving for an added touch of flavor and color.