Preheat the oven to 375°F (190°C). Grease a 9×13-inch baking dish with olive oil or nonstick spray.

Bring a large pot of salted water to a boil. Cook the lasagna noodles until al dente (usually 8–10 minutes). Drain and lay them flat to cool.

In a large skillet, heat 2 tablespoons olive oil over medium-high heat. Add the chopped onion and sauté for 3–4 minutes, until translucent.

Stir in the minced garlic and cook for about 1 minute until fragrant.

Add the sliced mushrooms, dried thyme, salt, and pepper. Cook until the mushrooms release their moisture and begin to brown, about 6–8 minutes.

Toss in the chopped spinach and cook until just wilted. Taste and adjust seasoning; remove from heat.

Spread a thin layer (about ½ cup) of marinara sauce on the bottom of the prepared baking dish.

Place 3 lasagna noodles over the sauce. Dollop and spread half the ricotta across the noodles. Spoon half the mushroom-spinach mixture over the ricotta. Sprinkle with about one-third of the mozzarella and one-third of the Parmesan.

Repeat the layers: sauce, 3 noodles, the remaining ricotta, the remaining mushroom-spinach mixture, and another third of the cheeses.

Finish with the last 3 noodles, the remaining marinara, and the remaining mozzarella and Parmesan.

Cover tightly with foil and bake for 25 minutes. Remove the foil and bake for another 10–15 minutes, until the top is bubbly and lightly golden.

Let the lasagna rest for 10–15 minutes before slicing. Garnish with fresh basil leaves and serve warm.