01 - Preheat the oven to 350°F (180°C). Lightly grease a 9 x 13 inch (23 x 33 cm) 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 using thawed frozen spinach, about 4 minutes. Season with salt. Drain excess moisture and roughly chop spinach.
03 - Combine ricotta, mozzarella, Parmesan, egg, ground nutmeg, salt, and black pepper in a bowl. Mix until smooth. Fold in 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 prevent lumps. Add salt, white pepper, and nutmeg. Cook while whisking until thick and creamy, approximately 8 minutes. Remove from heat.
05 - Spread a thin layer of béchamel sauce on the bottom of the baking dish. Add a layer of noodles, then one-third of the spinach-cheese mixture, followed by one-quarter of the béchamel sauce. Repeat layers twice more. Finish with a final noodle layer topped with remaining béchamel and sprinkle with extra Parmesan.
06 - Cover with foil tented to avoid cheese contact and bake for 30 minutes. Remove foil and bake an additional 15 to 20 minutes until golden and bubbling.
07 - Allow lasagna to rest for 15 minutes before slicing and serving.