Make a roux with the butter and flour. Add milk and beat with a whisk until smooth. Cook gently and season with salt, pepper and nutmeg.
Fry the finely chopped onion in oil. Add the leaf spinach and season with salt, pepper, garlic and nutmeg.
Lightly grease an ovenproof dish.
Fill the dish in layers with sheets of lasagne, béchamel sauce, spinach and little chunks of feta cheese. Pour on the béchamel sauce. Sprinkle with cheese.
Tip: instead of feta, you can also use mozzarella or ricotta.
