1. Preheat the Oven: Preheat your oven to 375°F. Grease a 9×13-inch baking dish to prepare for the layering process.

2. Cook the Noodles: Bring a large pot of salted water to a boil and cook the lasagna noodles until they are al dente. Drain the noodles and set them aside.

3. Prepare the Filling: In a skillet, heat the olive oil over medium heat. Sauté the onion until it becomes translucent. Add the minced garlic and cook for an additional minute. Then, add the sliced mushrooms, dried thyme, salt, and pepper. Cook the mixture until the mushrooms are tender. Finally, stir in the chopped spinach and cook until it’s wilted. Remove the skillet from the heat.

4. Assemble the Lasagna: Spread a thin layer of marinara sauce in the bottom of the prepared baking dish. Layer three lasagna noodles, followed by half of the ricotta cheese, half of the mushroom and spinach mixture, and one-third of the mozzarella and Parmesan cheeses. Repeat the layers, finishing with the remaining sauce and cheeses.

5. Bake the Lasagna: Cover the baking dish with foil and bake the lasagna for 25 minutes. Remove the foil and continue baking for an additional 10-15 minutes, or until the top is bubbly and golden brown.

6. Serve and Enjoy: Allow the lasagna to cool for a few minutes before serving. Garnish with fresh basil leaves and savor the flavors of this comforting, cheesy vegetarian dish.