1. Preheat your oven to 375°F. Grease a 9×13-inch baking dish.

2. Bring a large pot of salted water to a boil, then cook the lasagna noodles according to the package instructions until they reach an al dente texture. Drain the noodles and set them aside.

3. In a skillet, heat the olive oil over medium heat. Sauté the onion until it becomes translucent, then add the garlic and cook for an additional minute. Toss in the sliced mushrooms, dried thyme, salt, and pepper. Cook the mixture until the mushrooms are tender and have released their juices. Finally, stir in the chopped spinach and cook until it wilts.

4. Spread a thin layer of marinara sauce in the bottom of the prepared baking dish. Arrange three lasagna noodles over the sauce. Top with half of the ricotta cheese, half of the mushroom mixture, and one-third of the mozzarella and Parmesan cheeses. Repeat the layers, finishing with the remaining sauce and cheese.

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

6. Allow the Mushroom and Spinach Lasagna to rest for a few minutes before serving. Garnish with fresh basil leaves and savor the comforting, cheesy goodness of this vegetarian masterpiece.