Preheat your oven to 375°F (190°C) and grease a 9x13-inch baking dish with olive oil.

Boil a large pot of salted water, add the lasagna noodles, and cook until al dente, about 8-10 minutes.

In a large skillet, heat 2 tablespoons of olive oil over medium heat. Add chopped onion and sauté for 3-4 minutes until translucent, then add minced garlic and cook for another minute.

Incorporate sliced mushrooms, dried thyme, salt, and pepper into the skillet. Cook until mushrooms soften and liquid evaporates, about 5-7 minutes. Stir in spinach and cook until wilted, about 2-3 minutes.

In a medium bowl, combine ricotta cheese, salt, and pepper, mixing until smooth.

In the greased baking dish, spread a thin layer of marinara sauce, place three lasagna noodles on top, add dollops of the ricotta mixture, mushroom and spinach filling, and sprinkle mozzarella and Parmesan. Repeat until ingredients are used, finishing with marinara sauce and mozzarella.

Cover the lasagna with aluminum foil and bake for 25 minutes. Remove foil and bake for another 10-15 minutes until cheese is bubbly and golden brown.

Allow the lasagna to cool for about 10 minutes before serving. Garnish with fresh basil leaves.