1. Preheat your oven to 375°F (190°C).
2. Cook the lasagna noodles according to the package instructions. Drain and set aside.
3. In a large skillet, heat olive oil over medium heat. Add the mushrooms and cook until they release their juices and start to brown, about 5-7 minutes.
4. Add the garlic and spinach to the skillet and cook until the spinach wilts, about 2 minutes. Season with salt and pepper. Remove from heat and set aside.
5. In a saucepan, melt the butter over medium heat. Stir in the flour to create a roux, cooking for about 1-2 minutes until it turns golden.
6. Gradually whisk in the milk, stirring constantly until the sauce thickens, around 5-7 minutes. Stir in nutmeg, if using, and season with salt and pepper. Remove from heat.
7. Spread a thin layer of the bechamel sauce on the bottom of a 9×13 inch baking dish. Layer three lasagna noodles over the sauce.
8. Spread half of the ricotta cheese over the noodles, then add half of the mushroom-spinach mixture. Top with a third of the mozzarella and Parmesan cheeses.
9. Repeat the layers, ending with a layer of noodles and the remaining sauce. Sprinkle the rest of the mozzarella and Parmesan cheeses on top.
10. Cover with aluminum foil and bake for 25 minutes. Remove the foil and bake for an additional 15 minutes, or until the cheese is bubbly and golden.
11. Let the lasagna cool for about 10 minutes before slicing and serving. Enjoy!

Trust me, the wait is worth it. This resting time helps everything set up nicely for perfect slices.