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.

Add the minced garlic and sliced mushrooms to the skillet. Cook for another 5-7 minutes until the mushrooms are tender and any liquid has evaporated.

Stir in the fresh spinach, oregano, basil, salt, and pepper. Cook until the spinach is wilted, about 2-3 minutes. Remove from heat and let cool slightly.

In a mixing bowl, combine the ricotta cheese and egg. Mix until smooth.

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 mushroom and spinach mixture, and a third of the mozzarella cheese. Repeat the layers, starting with the noodles, then the ricotta, mushroom mixture, and another third of the mozzarella cheese.

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

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