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

Cook the lasagna noodles in salted boiling water until al dente, about 8–10 minutes. Drain and lay flat on a clean kitchen towel.

In a large skillet, heat olive oil over medium heat and sauté the chopped onion until translucent. Add minced garlic and cook for another minute.

Add sliced mushrooms, dried thyme, salt, and pepper to the skillet. Sauté until mushrooms are tender and liquid evaporates, then stir in spinach until wilted.

Spread marinara sauce on the bottom of the baking dish. Layer 3 lasagna noodles on top.

Spread half of the ricotta mixture over the noodles, followed by half of the mushroom-spinach filling and sprinkle with mozzarella and Parmesan. Repeat layers and finish with marinara and remaining cheeses.

Cover with foil and bake for 25 minutes, then remove the foil and bake for an additional 10–15 minutes until cheese is bubbly and golden brown.

Let the lasagna cool for about 10 minutes before slicing. Garnish with fresh basil and serve warm.