1. Preheat oven to 375°F (190°C) and grease a 9×13-inch baking dish.
2. In a skillet, sauté spinach with a pinch of salt until wilted. Set aside.
3. Prepare the white sauce: Melt butter in a saucepan over medium heat. Add garlic and sauté until fragrant. Stir in flour and cook for 1 minute. Gradually whisk in milk, stirring until thickened. Season with nutmeg (if using), salt, and pepper.
4. In a bowl, mix ricotta cheese with half the Parmesan, salt, and pepper.
5. Assemble the lasagna: Spread a thin layer of white sauce on the bottom of the baking dish. Add a layer of lasagna noodles, then ricotta mixture, sautéed spinach, and mozzarella. Repeat layers, finishing with noodles topped with white sauce and remaining Parmesan.
6. Cover with foil and bake for 25 minutes. Remove foil, bake for another 15 minutes, or until golden and bubbly.
7. Let rest for 10 minutes before slicing and serving.