Cut the onion into half rings. Peel the tomatoes and cut into thin slices. Grate the cheese on a fine grater. Crush the garlic with a knife. Cut the mozzarella into cubes. Preheat the oven to 180°C.
Heat a frying pan with vegetable oil and fry the crushed garlic until a characteristic aroma appears. Add onion and cook until soft. Add rosemary and bay leaf, then add tomatoes. Simmer the tomatoes covered for 30-35 minutes over medium heat.
Heat a separate frying pan with vegetable oil and fry the minced meat until golden brown, then pour in the wine. Wait until the wine has evaporated, and then simmer, covered, over medium heat for about 15 minutes.
Combine tomato sauce and prepared minced meat, mix well. Add tomato paste, salt, pepper and stir again. Simmer covered over low heat for about 15 minutes.
Melt the butter in a saucepan, add flour and mix well. Gradually add milk, stirring constantly, until sauce thickens, about 5 to 7 minutes. Grate the nutmeg.
Boil lasagna sheets in salted water until al dente.
Grease a baking dish in the oven with olive oil, arrange the lasagne sheets so they overlap each other, and spread the tomato sauce and mozzarella pieces on top. Repeat layers until you run out of ingredients. Place the remaining sauce on the last sheets of lasagna, pour in the bechamel sauce evenly and sprinkle with grated Parmesan.
Place the pan in the oven and bake the lasagna in an oven preheated to 180°C for 40-45 minutes.