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 3-4 minutes until translucent. Stir in the minced garlic and sliced mushrooms, cooking for another 5-7 minutes until the mushrooms are tender.

Add the fresh spinach to the skillet and cook until wilted, about 2-3 minutes. Season with salt, pepper, oregano, and basil. Remove from heat and let cool slightly.

In a mixing bowl, combine the ricotta cheese, egg, and half of the grated Parmesan cheese. Mix until smooth and well combined.

To assemble the lasagna, spread a thin layer of marinara sauce on the bottom of a 9x13 inch baking dish. Place 3 lasagna noodles over the sauce. Spread half of the ricotta mixture over the noodles, followed by half of the spinach and mushroom mixture, and a third of the mozzarella cheese. Repeat the layers, starting with the noodles, then the ricotta mixture, spinach and mushrooms, and mozzarella cheese.

Top the final layer of noodles with the remaining marinara sauce, the rest of the mozzarella cheese, and the remaining Parmesan cheese.

Cover the baking 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 brown.

Allow the lasagna to cool for about 10 minutes before slicing and serving.