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

2. Bring a pot of salted water to a boil and cook the lasagna noodles until al dente. Drain and set aside.

3. In a skillet, heat the olive oil over medium heat. Sauté the onion until translucent, then add the garlic and cook for 1 minute. Next, add the sliced mushrooms, thyme, salt, and pepper. Cook until the mushrooms are tender. Finally, stir in the chopped spinach and cook until wilted. Remove from heat.

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

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

6. Allow the lasagna to cool for a few minutes, then garnish with fresh basil leaves and serve warm.