Preheat your oven to 375°F (190°C). 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 for about 3 minutes until translucent. Stir in the minced garlic and sliced mushrooms, cooking for another 5 minutes until the mushrooms are tender.
Add the fresh spinach to the skillet and cook until wilted, about 2 minutes. Remove from heat and let cool slightly.
In a mixing bowl, combine the ricotta cheese, grated Parmesan cheese, egg, dried oregano, salt, and black pepper. Mix well.
Spread a thin layer of marinara sauce on the bottom of a 9x13 inch baking dish. Layer 3 lasagna noodles over the sauce.
Spread half of the ricotta mixture over the noodles, followed by half of the spinach and mushroom mixture. Sprinkle with 1 cup of shredded mozzarella cheese.
Repeat the layering process: add another layer of noodles, the remaining ricotta mixture, the remaining spinach and mushroom mixture, and another cup of mozzarella cheese. Top with the final layer of noodles and the remaining marinara sauce.
Finish by sprinkling the remaining mozzarella cheese on top. Cover the dish with aluminum foil and bake for 25 minutes.
Remove the foil and bake for an additional 15 minutes, or until the cheese is bubbly and golden. Let it cool for 10 minutes before slicing.
Garnish with fresh basil if desired before serving.