Preheat the oven to 375°F (190°C).
In a large pot of salted boiling water, cook the lasagna noodles according to package instructions until al dente. Drain and set aside.
In a large skillet, heat the olive oil over medium heat. Add the diced onion and sauté until translucent, about 5 minutes.
Add the sliced mushrooms to the skillet and cook for another 5 minutes until browned. Stir in the minced garlic and cook for an additional minute.
Add the chopped spinach, oregano, basil, salt, and pepper. Cook until the spinach wilts, about 2 minutes. Remove from heat and let it cool slightly.
In a mixing bowl, combine the ricotta cheese, 1 cup of mozzarella cheese, and Parmesan cheese. Mix well.
To assemble the lasagna, spread a thin layer of marinara sauce on the bottom of the baking dish. Place 3 lasagna noodles on top, followed by half of the ricotta mixture, half of the mushroom and spinach mixture, and another layer of marinara sauce. Repeat the layers, finishing with the noodles and a final layer of marinara sauce on top.
Sprinkle the remaining mozzarella cheese on top of the lasagna.
Cover the baking dish with aluminum foil and bake for 30 minutes. Remove the foil and bake for an additional 15 minutes or until the cheese is bubbly and golden brown.
Allow the lasagna to rest for 10 minutes before slicing. Garnish with fresh basil leaves if desired.