Preheat the oven to 375°F (190°C).

Cook the 12 lasagna noodles in boiling water with salt until al dente, about 8-10 minutes. Drain and lay flat.

In a skillet, heat olive oil and sauté onion for 5 minutes. Add garlic and mushrooms, cooking until browned.

Add spinach to the skillet and cook until wilted. Season with oregano, basil, salt, and pepper. Add nutmeg if desired. Remove from heat.

In a bowl, mix ricotta cheese, egg, mozzarella, and half of the Parmesan until smooth.

Spread ½ cup marinara sauce in a baking dish. Layer 4 noodles, half of the cheese mixture, half of the vegetable mixture, and a third of the marinara. Repeat the layers.

Top with remaining noodles, marinara, mozzarella, and Parmesan. Cover with foil.

Bake covered for 25 minutes, then uncover and bake for 15 more until bubbly and golden.

Let cool for 10-15 minutes, then cut and serve.