First infuse the milk for the béchamel sauce. Put the milk, onion, bay and cloves into a large saucepan and bring very gently just up to the boil. Turn off the heat and set aside for 1 hr to infuse.
For the meat sauce, put the oil, celery, onion, carrot, garlic and pancetta in another large saucepan. Gently cook together until the veg are soft but not coloured. Tip in all the mince, the milk and tomatoes. Using a wooden spoon, stir together and break up and mash the lumps of mince against the sides of the pan. When the mince is mostly broken down, stir in all the herbs, the stock cubes and wine, and bring to a simmer. Cover and cook for 1 hr, stirring occasionally to stop the bottom catching. Uncover and gently simmer for another 30 mins-1 hr until the meat is tender and saucy. Taste and season.
To finish the béchamel, strain the milk through a fine sieve into one or two jugs. Melt the butter in the same pan then, using a wooden spoon, mix in the flour and cook for 2 mins. Stir in the strained milk, a little at a time – the mix will thicken at first to a doughy paste, but keep going, adding milk gradually to avoid lumps. When all the milk is in, bring to a gentle simmer, stirring constantly (if you have lumps, give it a quick whisk). Gently bubble for a few mins until thickened. Season with salt, pepper and nutmeg.
Heat oven to 180C/160C fan/gas 4. Spread a spoonful of the meat sauce over the base of a roughly 3.5 litre baking dish. Cover with a single layer of pasta sheets, snapping them to fit if needed, then top with a quarter of the béchamel. Spoon over a third of the meat sauce and scatter over a little Parmesan. Repeat the layers – pasta, béchamel, meat and Parmesan – two more times to use all the meat sauce. Add a final layer of pasta, the last of the béchamel and remaining Parmesan. Sit the dish on a baking tray to catch spills and bake for 1 hr until bubbling, browned and crisp on top.