Heat 2 tablespoons of olive oil in a large skillet over medium heat. Once hot, add 8 ounces of sliced mushrooms and sauté for about 5 minutes until they're golden brown and tender. Stir in 3 cups of fresh spinach and 2 minced garlic cloves, cooking until the spinach has wilted. Season with salt and pepper to taste, then remove from heat and set aside to cool slightly.

In a saucepan, melt 4 tablespoons of butter over medium heat. Once melted, whisk in 1/4 cup of all-purpose flour to create a roux, cooking for 1-2 minutes until lightly golden. Gradually whisk in 2 cups of warm milk, and bring to a simmer while stirring continuously to avoid lumps. Cook for about 5-7 minutes until thickened, then add a pinch of nutmeg, salt, and pepper to enhance the flavor.

If using traditional lasagna noodles, cook them according to package instructions until al dente, then drain and rinse under cool water. For no-boil noodles, you can skip this step as they will soften during baking.

Preheat your oven to 375°F (190°C). In a 9x13-inch baking dish, spread a thin layer of the béchamel sauce to coat the bottom. Place a layer of lasagna noodles over the sauce, followed by half of the sautéed mushroom and spinach filling. Dollop with one cup of ricotta cheese and sprinkle a mixture of mozzarella and parmesan cheese on top. Repeat the layers, finishing with the remaining béchamel and a generous sprinkle of mozzarella cheese.

Cover the assembled lasagna with aluminum foil to prevent over-browning. Bake in the preheated oven for 25 minutes. After this time, carefully remove the foil and continue baking for another 15 minutes, or until the top is golden and bubbling beautifully.

Once baked, remove the lasagna from the oven and allow it to rest for about 10 minutes. This resting period helps the layers set, making it easier to cut and serve. Slice into squares, and serve warm.