In a large skillet, heat 2 tablespoons of olive oil over medium heat. Add sliced mushrooms and sauté for about 5 minutes until they turn golden brown and slightly crispy. Then, stir in 2 cups of fresh spinach and 2 minced garlic cloves, cooking until the spinach wilts. Season the mixture with salt and pepper to taste, then remove from heat and set aside.

In a medium saucepan, melt 4 tablespoons of butter over low heat. Whisk in ¼ cup of all-purpose flour, forming a smooth paste, and cook for 2 minutes to eliminate the raw flour taste. Gradually whisk in 3 cups of warm milk and cook the mixture, stirring constantly, until it thickens and coats the back of a spoon (about 5-7 minutes). Finish by adding a pinch of ground nutmeg, plus salt and pepper as needed.

Preheat your oven to 375°F (190°C). In a greased 9×13-inch baking dish, spread a thin layer of the béchamel sauce on the bottom. Place three lasagna noodles over the sauce, then layer ⅓ of the vegetable filling and dollops of 1 cup of ricotta cheese, followed by a sprinkle of 1 cup of mozzarella cheese. Repeat this layering process twice, finishing with a final layer of béchamel and topping it with the remaining mozzarella and ¼ cup of grated Parmesan cheese.

Cover the assembled lasagna with aluminum foil to prevent it from drying out. Bake it in the preheated oven for 25 minutes. After this time, carefully remove the foil and bake for an additional 15 minutes, allowing the cheese to bubble and turn a lovely golden brown. Keep an eye on it to avoid over-browning.

Once baked, remove the lasagna from the oven and let it rest for 10 minutes before slicing. This resting time allows the layers to set, making it easier to serve slices without them falling apart. Serve warm, and relish the comforting flavors of this Creamy Spinach Mushroom Lasagna!