1. Preheat your oven to 375°F (190°C). Lightly grease a 9×13-inch baking dish.
2. Boil the lasagna noodles in salted water until al dente, about 8 minutes. Drain and set aside.
3. In a large skillet, heat olive oil over medium heat. Add mushrooms and cook until they release their juices and begin to brown, approximately 5 minutes.
4. Add garlic and spinach to the skillet, cooking until the spinach wilts. Season with salt, pepper, and thyme. Remove from heat and set aside.
5. In a saucepan, melt butter over medium heat. Stir in flour to create a roux, then gradually whisk in milk. Cook until the mixture thickens, about 5-7 minutes. Stir in Parmesan cheese until melted and smooth. Season with a pinch of nutmeg if desired.
6. Spread a thin layer of the béchamel sauce at the bottom of the baking dish. Layer 3 lasagna noodles over the sauce.
7. Spread half of the mushroom and spinach mixture over the noodles, followed by half of the ricotta cheese.
8. Repeat the layers: béchamel, noodles, mushroom-spinach mixture, and ricotta. Finish with a final layer of noodles and pour remaining béchamel sauce over the top.
9. Sprinkle mozzarella cheese evenly over the lasagna.
10. Bake in the preheated oven for 30-35 minutes, or until the cheese is golden and bubbly. Let it rest for 10 minutes before serving.

Pro tip from my kitchen to yours: let the lasagna rest before cutting into it. This allows the flavors to meld beautifully, and you’ll get those perfect slices every time.