Preheat Oven
Preheat your oven to 375°F (190°C).

Cook Noodles
Boil lasagna noodles according to package directions until al dente (about 8–10 minutes). Drain and set aside.

Sauté Vegetables
In 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 Spinach
Stir in fresh spinach and cook until wilted (about 2 minutes). Remove from heat and let cool slightly.

Mix Cheese Filling
In a bowl, combine ricotta, Parmesan, egg, oregano, salt, and black pepper. Stir until smooth and fully combined.

Assemble Lasagna
In 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.

Bake
Cover with aluminum foil and bake for 25 minutes. Remove foil and bake another 15–20 minutes until cheese is golden and bubbly.

Cool and Serve
Let the lasagna rest for 10 minutes before slicing and serving.