For tomato sauce: cook the diced onion in a tablespoon of oil, cut the tomatoes into cubes and add them. Stir in sugar, bay leaves and tomato concentrate. Cook for at least 15 minutes.
For the bolognese sauce: cook the minced meat in the tomato sauce (already prepared) for 5 minutes
For the bechamel sauce: melt the butter in a saucepan, add the flour and stir energetically. Add the milk little by little while continuing to whisk. When the preparation becomes creamy, remove from heat and add salt and pepper.
Butter a rectangular mussel and arrange a coat of lasagne, spread above a layer of bolognese sauce. Repeat the same operation three times, end with a layer of bechamel and sprinkle with grated Parmesan. Cook in 180° for 45 mn.