Heat olive oil in a large skillet over medium heat. Sauté diced onion for 3-4 minutes until translucent.
Add minced garlic and cook for 1 minute. Stir in sliced mushrooms and cook for 5-7 minutes until tender.
Add fresh spinach, dried oregano, dried basil, salt, and pepper. Cook until spinach wilts, about 2 minutes. Remove from heat.
In a medium bowl, combine ricotta cheese, egg, salt, and pepper. Mix until smooth.
Spread a thin layer of marinara sauce in a 9x13 inch baking dish. Layer 3 lasagna noodles over the sauce.
Add half of the ricotta mixture, half of the mushroom and spinach filling, and a third of the shredded mozzarella cheese. Repeat layers.
Finish with a last layer of noodles topped with marinara sauce, remaining mozzarella, and grated Parmesan cheese.
Cover with aluminum foil and bake in a preheated oven at 375°F for 25 minutes. Remove foil and bake for an additional 15 minutes.
Let cool for 10 minutes before slicing and serving.