Preheat oven to 375°F (190°C).
Cook lasagna noodles as per package instructions, then drain and set aside.
Heat olive oil in a pan, sauté mushrooms until tender. Add spinach and cook until wilted. Season with salt and pepper.
In a bowl, mix ricotta, egg, and half the mozzarella and Parmesan. Set aside.
Melt butter in a saucepan for the béchamel. Add flour and cook for 2 minutes. Gradually whisk in milk until smooth. Add garlic powder, onion powder, nutmeg, salt, and pepper. Cook until thickened.
Spread a thin layer of béchamel in a baking dish. Layer noodles, ricotta mixture, vegetables, and more sauce. Repeat layers, finishing with noodles and sauce on top.
Sprinkle remaining mozzarella and Parmesan on top.
Bake for 30 minutes until golden and bubbly. Let rest for 10 minutes before serving.
Garnish with parsley if desired.