Preheat OvenPreheat your oven to 375°F (190°C).
Cook NoodlesBoil lasagna noodles according to package directions until al dente (about 8–10 minutes). Drain and set aside.
Sauté VegetablesIn a large skillet, heat olive oil over medium heat. Add diced onion and sauté for 3–4 minutes until translucent. Add garlic and mushrooms; cook for 5–7 minutes until mushrooms are tender and browned.
Add SpinachStir in fresh spinach and cook until wilted (about 2 minutes). Remove from heat and let cool slightly.
Mix Cheese FillingIn a bowl, combine ricotta, Parmesan, egg, oregano, salt, and black pepper. Stir until smooth and fully combined.
Assemble LasagnaIn a 9×13-inch baking dish, spread a thin layer of marinara sauce. Add 3 noodles, half of the ricotta mixture, half of the mushroom-spinach mixture, and ⅓ of the mozzarella. Repeat layers. Top with final 3 noodles, remaining sauce, and the rest of the mozzarella.
BakeCover with aluminum foil and bake for 25 minutes. Remove foil and bake another 15–20 minutes until cheese is golden and bubbly.
Cool and ServeLet the lasagna rest for 10 minutes before slicing and serving.