Preheat your oven to 375°F (190°C). Grease a 9x13-inch baking dish with cooking spray or olive oil.

Bring a large pot of salted water to a boil. Add lasagna noodles and cook until al dente according to package instructions, usually 8-10 minutes. Drain, rinse with cool water, and set aside on a clean kitchen towel.

Heat olive oil in a large skillet over medium heat. Add the finely chopped onion and sauté until translucent and fragrant, about 3-4 minutes. Add minced garlic and cook for another minute until aromatic.

Add sliced mushrooms, dried thyme, salt, and pepper to the skillet. Cook for 6-8 minutes, stirring occasionally, until mushrooms are tender and golden brown and any excess liquid has completely evaporated.

Add chopped spinach leaves to the skillet and cook until wilted, about 2-3 minutes. The spinach will reduce significantly. Remove from heat and set aside.

Spread a thin layer of marinara sauce (about 1/2 cup) on the bottom of the prepared baking dish. Place three lasagna noodles side by side on top of the sauce.

Spread half of the ricotta cheese evenly over the noodles, followed by half of the mushroom-spinach mixture. Sprinkle with about 1/3 cup each of the mozzarella and Parmesan cheeses.

Repeat the layering process with three more noodles, remaining ricotta, remaining mushroom mixture, and another 1/3 cup each of the cheeses. Top with the final three noodles and remaining marinara sauce, then finish with the rest of the mozzarella and Parmesan.

Cover the baking dish tightly with aluminum foil and bake for 25 minutes. Remove foil and bake for an additional 10-15 minutes until the cheese on top is bubbly and golden brown.

Let the lasagna rest for 10-15 minutes before slicing to allow the layers to set. Garnish with fresh basil leaves and serve.