Prepare the Béchamel Sauce:

1. Make the Roux: In a large saucepan, melt the butter over medium heat. Add the flour and whisk continuously to create a smooth roux. Cook for 1-2 minutes until golden, but not browned.
2. Add Milk: Gradually whisk in the milk, ensuring no lumps form. Bring the mixture to a simmer and cook for 5-7 minutes, whisking frequently, until the sauce thickens and coats the back of a spoon.
3. Season the Sauce: Stir in the dried thyme or Italian seasoning, salt, pepper, and a pinch of nutmeg if using. Remove from heat and set aside.

Prepare the Vegetables:

4. Sauté the Mushrooms: In a large skillet, heat olive oil over medium-high heat. Add the sliced mushrooms and cook for 5-7 minutes, stirring occasionally, until softened and browned. Season with salt and pepper.
5. Add the Spinach: Add the chopped spinach to the mushrooms and cook for another 2-3 minutes, or until wilted. Remove from heat and set aside.

Assemble the Lasagna:

6. Cook the Lasagna Noodles: If using regular lasagna noodles, cook them according to package instructions until al dente. Drain and set aside. If using no-boil noodles, skip this step.
7. Layer the Lasagna: Preheat your oven to 375°F (190°C). Spread a thin layer of béchamel sauce in the bottom of a 9×13-inch baking dish. Place a layer of lasagna noodles over the sauce, then add a third of the mushroom and spinach mixture, followed by a third of the ricotta cheese, a third of the mozzarella cheese, and a third of the béchamel sauce. Repeat the layers two more times, ending with a layer of béchamel sauce and a generous sprinkle of Parmesan cheese on top.
8. Bake the Lasagna: Cover the lasagna with aluminum foil and bake in the preheated oven for 40 minutes. Remove the foil and bake for an additional 10-15 minutes, or until the top is golden and bubbling.

Serve the Lasagna:

9. Rest and Serve: Let the lasagna rest for 10 minutes before slicing and serving. Garnish with fresh basil if desired.