1. Preheat the oven to 180 degrees.
  2. Boil the spinach over medium heat with enough water. Drain and finely chop it.
  3. Heat the olive oil and fry the onion, after which add the spinach and cook for 2 more minutes. Season with salt and pepper.
  4. For the sauce: melt some butter in a small dish and mix it with the flour. Slowly pour the milk as you try to avoid any lumps and let the mixture boil for 1 minute over a medium heat. Season with salt, pepper and nutmeg.
  5. Grease a baking dish and alternate: a sheet of lasagna, spinach mixture, cheese and sauce.
  6. Finish up the last layer with grated yellow cheese (instead of feta cheese) and sauce and baked the meal in the oven for 30 minutes (until its crust is golden brown).
  7. When serving place a few basil leaves on top.