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

Bring a large pot of salted water to a rolling boil and cook the lasagna noodles according to package instructions until al dente, about 8-10 minutes. Drain and rinse under cold water.

In a large skillet, heat about 2 tablespoons of olive oil over medium heat. Add finely chopped onion and sauté for 3-4 minutes until it becomes translucent.

Add minced garlic and sliced mushrooms to the skillet and cook for another 5-7 minutes until mushrooms are tender.

Stir in chopped spinach and cook for 2-3 minutes until it wilts. Remove from heat and let the filling cool slightly.

Spread a thin layer of marinara sauce in the greased baking dish. Lay three lasagna noodles over the sauce.

Spread half of the ricotta cheese over the noodles, followed by half of the mushroom and spinach mixture, and a third of the mozzarella and Parmesan.

Repeat layering with another set of noodles, remaining ricotta, mushroom mixture, and remaining cheeses, finishing with marinara sauce on top.

Cover the dish with aluminum foil and bake for 25 minutes. Remove the foil and bake for an additional 10-15 minutes until cheese is bubbling and golden.

Let the lasagna cool for about 10 minutes before slicing. Garnish with fresh basil if desired.