Finely dice the onion and fry in 1 tablespoon hot oil over medium heat until translucent. Add spinach and let thaw over low heat, stirring every now and then. Season the spinach with salt, pepper and a pinch of nutmeg.
Melt 2 tablespoons butter in a small saucepan, add 3 tablespoons flour and stir well with a whisk. Turn off the stove and gradually add the milk while stirring. Stir until there are no more lumps or residues on the bottom. Season the sauce with 1 teaspoon salt, 2 pinches of pepper and 2 - 3 knife tips of grated nutmeg.
Cut the mozzarella into slices and preheat the oven to 160 degrees.
Layer the lasagna, at the very bottom a thin layer of spinach and then always according to the following pattern: Lasagne sheets - spinach - 6 tablespoons of sauce.
Finally pour the rest of the sauce on top and put the mozzarella on top. Sprinkle the mozzarella with salt and cover the baking dish with aluminum foil. Cook for 25 minutes on the middle rack with aluminum foil and then for 20 minutes without aluminum foil. When the mozzarella is golden brown in some places, the lasagna is ready.