Step 1: Prepare the Béchamel Sauce

1. In a saucepan, melt the butter over medium heat.
2. Stir in the flour and cook for 1-2 minutes until lightly golden.
3. Gradually whisk in the milk, making sure to avoid lumps.
4. Add nutmeg, salt, and pepper, then let the sauce simmer until thickened. Remove from heat and set aside.

Step 2: Sauté the Mushrooms and Spinach

1. Heat olive oil in a large pan over medium heat.
2. Add the chopped onion and garlic, cooking until softened.
3. Stir in the mushrooms and cook until they release their juices and become tender.
4. Add the spinach and cook until wilted. Season with oregano, red pepper flakes, salt, and pepper. Remove from heat.

Step 3: Cook the Lasagna Noodles

1. Boil lasagna noodles according to package instructions until al dente.
2. Drain and set aside.

Step 4: Assemble the Lasagna

1. Preheat oven to 375°F (190°C).
2. Spread a thin layer of béchamel sauce on the bottom of a 9×13-inch baking dish.
3. Lay three lasagna noodles on top.
4. Spread half of the ricotta cheese over the noodles, followed by half of the mushroom-spinach mixture.
5. Sprinkle with Parmesan and mozzarella cheese.
6. Repeat the layers, finishing with noodles and a generous topping of béchamel and cheese.

Step 5: Bake the Lasagna

1. Cover with foil and bake for 30 minutes.
2. Remove foil and bake for another 10-15 minutes, until the top is golden and bubbly.

Step 6: Let It Rest and Serve

1. Allow the lasagna to rest for 10 minutes before slicing.
2. Garnish with fresh basil or parsley and serve warm.