1. Preheat the Oven:

Preheat your oven to 375°F (190°C). Lightly grease a 9×13-inch baking dish with olive oil or butter.

2. Prepare the Filling:

Heat olive oil or butter in a large skillet over medium heat. Add the finely chopped onion and cook until soft and translucent, about 3 minutes. Stir in the minced garlic and cook for an additional minute until fragrant.

Next, add the sliced mushrooms and cook until they are browned and any liquid has evaporated, about 7-8 minutes. Lastly, fold in the fresh spinach, dried thyme, salt, and pepper. Cook until the spinach is wilted (or warmed through if using frozen). Remove from heat and set aside.

3. Make the Béchamel Sauce:

In a medium saucepan, melt the butter over medium heat. Sprinkle in the flour and whisk continuously for 1-2 minutes to create a roux. Gradually add the warmed milk, whisking constantly to prevent lumps from forming. Cook until the sauce thickens (about 5-7 minutes). If you choose to use nutmeg, season your béchamel with it, along with a pinch of salt and freshly cracked black pepper. Remove from heat.

4. Assemble the Lasagna:

Spread a thin layer of béchamel sauce on the bottom of the baking dish to prevent sticking. Layer 3-4 lasagna noodles on top, slightly overlapping them. Spread 1/3 of the spinach and mushroom mixture over the noodles, followed by 1/3 of the béchamel sauce. Sprinkle with 1/3 of the mozzarella and Parmesan cheese. Repeat the layers two more times, finishing with a layer of béchamel sauce and a generous topping of cheese.

5. Bake:

Cover the lasagna with foil and bake for 25 minutes. After that, remove the foil and bake for an additional 15 minutes, or until the top is golden and bubbly.

6. Rest and Serve:

Let the lasagna rest for 10 minutes before slicing. This rest time allows the flavors to meld beautifully. Serve warm, garnished with freshly chopped herbs if desired.