In a large skillet, heat 2 tablespoons of olive oil over medium heat. Add the sliced mushrooms and sauté for about 5-7 minutes until they are soft and browned. Stir in the minced garlic and chopped spinach, cooking until the spinach has wilted, about 2-3 minutes. Season with salt and pepper to taste, then remove from heat and set aside.

In a medium saucepan, melt 3 tablespoons of butter over low heat. Whisk in 3 tablespoons of all-purpose flour to form a smooth paste, cooking for about 1-2 minutes. Gradually pour in 3 cups of warm milk while continuously whisking, cooking until thickened, for approximately 5-7 minutes. Season with salt, pepper, and ½ teaspoon of ground nutmeg, then remove from heat.

Preheat your oven to 375°F (190°C). In a 9x13 inch baking dish, layer a small amount of béchamel sauce on the bottom. Place 3 lasagna noodles, layer sautéed vegetable filling, dollops of 1 cup of ricotta cheese, and a sprinkle of 1 cup of shredded mozzarella. Repeat layers, finishing with béchamel sauce and remaining mozzarella and ½ cup grated Parmesan on top.

Cover the lasagna with aluminum foil, ensuring it doesn't touch the cheese. Bake for 25 minutes, then remove the foil and bake for another 15 minutes, or until the top is golden brown and bubbly.

Once baked, remove the lasagna from the oven and let it rest for about 10 minutes before serving warm, and enjoy!