To make the meat sauce, heat the oil in a wide saucepan and cook the onion, garlic, carrot and celery on low heat until soft. Increase the heat to medium, stir in the beef and bacon, breaking up any lumps with two forks, and cook, stirring, until the meat changes colour. Add the wine, tomatoes, oregano and nutmeg, cover and simmer for 30 minutes. Season the sauce to taste with salt and pepper.
To make the béchamel sauce, heat the milk to simmering point with the onion and bouquet garni, then remove from the heat and stand for 10 minutes to infuse. Heat the butter in a small saucepan, stir in the flour and cook, stirring, for 3 minutes. Remove from the heat and allow to cool a little. Strain the warm milk and stir it into the flour mixture until blended. Stir on low heat until the sauce boils and thickens. Season with salt and pepper.
Preheat the oven to 180°C. Grease a shallow, rectangular ovenproof dish. Spread a layer of meat sauce on the base of the dish and top with a layer of béchamel sauce. Add a layer of mozzarella, then a layer of pasta. Repeat these four layers until the dish is full, ending with béchamel sauce. Sprinkle with Parmesan cheese and bake for 40 minutes, or until the lasagne is hot and bubbling. Stand for 10 minutes before serving. Serve with a green salad and crusty bread.