Clean and chop the onions finely. Pour the olive oil in a pan and heat it on the stove.
Stir again and leave the sauce to boil until the excess liquid evaporates and the sauce thickens. Boil the macaroni in 1 gal (4 L) salted water according to package directions.
Put the butter in a casserole on the stove, melt it, pour in the flour, stir with a whisk and braise for 1 min. until golden. Pour in the milk and stir until you get a medium thick sauce. Season with salt, black pepper and a pinch of nutmeg.
Strain the macaroni, smear a deep oven dish with a little butter and pour in half the macaroni. Pour on half the Bechamel sauce, then half the tomato sauce and sprinkle with the grated ripe mozzarella.
Cover with the other half of macaroni, the remaining Bechamel and the tomato sauce on top. Drain the mozzarella from the brine and cut it into pieces. Arrange the mozzarella on top of the sauce.
Heat the oven to 356°F (180 °C) and put the oven dish in it. Bake the macaroni for about 20 min. until the mozzarella melts. Take it out and serve sprinkled with a little parsley.
If you'd like to serve a nice firm piece, wait for it to cool. After the lasagna cools it firms up and looks better visually but if you can't wait - eat it warm like I did!