01 - Preheat oven to 350°F. Lightly grease a 9 x 13-inch baking dish with olive oil.
02 - Heat olive oil in a large skillet over medium heat. Add minced garlic and sauté for 30 seconds. Add spinach and cook until wilted or heated through if frozen, about 4 minutes. Season with salt, then drain excess moisture in a colander and roughly chop.
03 - In a bowl, mix ricotta, mozzarella, Parmesan, egg, nutmeg, salt, and black pepper until smooth. Fold in the chopped spinach.
04 - Melt butter in a saucepan over medium heat. Stir in flour and cook for 1 minute. Gradually whisk in milk, stirring constantly to avoid lumps. Add salt, white pepper, and nutmeg. Cook until thick and creamy, about 8 minutes, then remove from heat.
05 - Spread a thin layer of béchamel sauce on the bottom of the baking dish. Layer noodles over sauce, then spread one-third of the spinach-cheese mixture, followed by one-fourth of the béchamel sauce. Repeat layers two more times, finishing with noodles topped by the remaining béchamel. Sprinkle extra Parmesan evenly over the top.
06 - Cover with foil tented to prevent cheese contact and bake for 30 minutes. Remove foil and bake an additional 15–20 minutes until golden and bubbling.
07 - Allow to stand for 15 minutes before slicing and serving.