Preheat your oven to 375°F (190°C).
Prepare the béchamel sauce by melting butter in a saucepan and whisking in flour until pale.
Gradually whisk in milk, stirring constantly until the sauce thickens.
Season the béchamel with nutmeg, salt, and pepper, then set aside.
Sauté garlic until fragrant, then add the spinach and cook until wilted.
In a baking dish, spread a thin layer of béchamel sauce on the bottom, followed by noodles.
Layer with ricotta, mozzarella, sautéed spinach, and béchamel, then top with noodles.
Repeat layering until all ingredients are used, finishing with béchamel and a generous sprinkle of Parmesan.
Cover with foil and bake for 25 minutes, then uncover and bake for an additional 15-20 minutes until golden.
Let the lasagna rest for 10-15 minutes before slicing and serving.