Prepare the Vegetable Filling: Heat 2 tablespoons of olive oil in a large skillet over medium heat. Add the sliced mushrooms and sauté for about 5 minutes until they turn golden brown. Toss in the fresh spinach and minced garlic, cooking until the spinach wilts down. Season with salt and pepper to taste, then remove from heat and set aside.

Make the Béchamel Sauce: In a saucepan, melt 3 tablespoons of butter over medium heat until bubbling. Stir in 3 tablespoons of all-purpose flour to create a roux, cooking for 1-2 minutes. Gradually add 3 cups of warm milk, whisking thoroughly. Bring to a gentle simmer, stirring until thickened. Finish by seasoning with ½ teaspoon of ground nutmeg, salt, and pepper.

Assemble the Lasagna: Preheat your oven to 375°F (190°C). Spread a thin layer of béchamel sauce in a baking dish. Layer 3 lasagna noodles, followed by ⅓ of the vegetable filling, dollops of ricotta, a sprinkle of mozzarella, and a scoop of béchamel. Repeat twice more, finishing with a final layer of béchamel, topped with remaining mozzarella and Parmesan.

Bake the Lasagna: Cover tightly with aluminum foil and bake for 25 minutes. Remove the foil and bake for another 15 minutes or until bubbly and golden. Keep an eye on it to ensure it doesn’t brown too quickly.

Serve: Let the lasagna rest for about 10 minutes before slicing. Serve warm portions and enjoy!