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 olive oil over medium heat. Add diced onion and sauté until translucent, about 5 minutes.
Add the minced garlic and sliced mushrooms to the skillet. Cook until the mushrooms are browned, about 8 minutes.
Stir in the fresh spinach and cook until wilted, about 2 minutes. Remove from heat and let cool slightly.
In a mixing bowl, combine the ricotta cheese, egg, oregano, salt, and pepper. Mix well.
Spread a thin layer of marinara sauce on the bottom of the baking dish. Lay 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: noodles, ricotta mixture, mushroom and spinach mixture, and more marinara sauce. Top with the remaining noodles, marinara sauce, and the rest of the mozzarella cheese and grated Parmesan.
Cover the baking dish with aluminum foil and bake for 25 minutes.
Remove the foil and bake for an additional 20 minutes or until the cheese is bubbly and golden.
Let the lasagna cool for about 10 minutes before slicing and serving.