Preheat your oven to 375°F (190°C).

In a large pot, bring salted water to a boil and cook the lasagna noodles according to package directions. Drain and set aside.

In a large skillet, heat the olive oil over medium heat. Add the chopped onion and minced garlic, cooking until softened, about 3 minutes.

Stir in the zucchini and mushrooms, cooking until they are soft, around another 5 minutes. Finally, add the fresh spinach and cook until wilted. Season with salt, pepper, oregano, and basil.

In a saucepan, melt 4 tablespoons of butter over medium heat. Whisk in the flour to create a roux and cook for a minute.

Gradually whisk in the milk, and continue to stir until the sauce thickens, about 5-7 minutes. Season with salt and pepper.

In a baking dish, spread a thin layer of the sauce at the bottom. Place 4 lasagna noodles on top, then layer half of the vegetable mixture, followed by half of the ricotta cheese, and a layer of mozzarella.

Repeat the layering process, finishing with noodles on top. Pour the remaining sauce and top with grated Parmesan and mozzarella.

Cover the baking dish with aluminum foil and bake for 25 minutes. Remove the foil and bake for an additional 15 minutes or until the top is golden and bubbly. Let rest for 10 minutes before slicing and serving.