Melt the butter in a medium saucepan over medium heat. Whisk in the flour to create a roux, cooking for about 2 minutes until light golden. Gradually whisk in the milk, stirring constantly until it thickens and bubbles, about 5–7 minutes. Add nutmeg and remove from heat.

In a large skillet, heat olive oil over medium heat. Add minced garlic and sauté until fragrant, about 1 minute. Toss in sliced mushrooms and cook until browned, around 5 minutes. Add fresh spinach and stir until just wilted, approximately 2 minutes, then remove from heat.

Preheat your oven to 375°F (190°C). Spread a thin layer of béchamel sauce in a baking dish. Layer lasagna noodles over the sauce, followed by half of the mushroom and spinach mixture. Sprinkle with cheese and repeat until all ingredients are used, finishing with béchamel and cheese on top.

Cover the dish with aluminum foil and bake for 30 minutes. Remove foil and bake uncovered for an additional 15 minutes, or until golden brown and bubbling.

Let the lasagna rest for about 10 minutes before slicing. Serve warm, optionally garnished with fresh herbs.