Heat the butter in a large saucepan on a low heat. Tip in the pancetta and cook for 5 minutes until the fat is starting to melt, then scoop it out with a slotted spoon and put it to one side. Add the finely chopped onion, carrot and celery to the pan. Cook very gently for 15 minutes, stirring often, to soften and sweeten the vegetables without them browning.
Once the vegetables are soft, add the pancetta back to the pan and crumble in the minced beef and pork, then add the salt and pepper. Turn up the heat to medium and carry on cooking, stirring the meat around for 5-10 minutes until any liquid has evaporated and there is a sticky brown crust on the bottom of the pan - this will create extra flavour! Add the white wine and use a wooden spatula to scrape up the crust from the bottom of the pan, then cook for another few minutes until the wine evaporates.
Preheat the oven to 140 C. Add the tinned tomatoes and water to the pan. Mix them through and bring the sauce to a simmer, then put the lid on and put into the oven for 3 hours. Check the pot occasionally, giving it a stir and adding another 100 millilitres of water if it starts to look dry - although it shouldn’t.
After 3 hours, bring the pot out of the oven, remove the lid and add the milk. Cook on a low heat, uncovered, for 5-10 minutes to reduce any liquid, if necessary. You want a thick, meaty ragu.
Make the bechamel: Melt the butter in a large saucepan then whisk in the flour and cook for 2 minutes. Switch off the heat while you gradually whisk in the milk, until smooth. Turn the heart back on and cook, whisking constantly, while the sauce thickens. Add in the parmesan and a large pinch of nutmeg and mix until the cheese has melted. Then take off the heat.
Assemble the lasagne: Preheat the oven to 200C / 180C fan.
Wipe the inside of the serving dish with olive oil. Put a quarter of the ragu mixture into the dish and spread it out evenly. Pour over one-fifth of the bechamel and spread it around, then top with 2 lasagne sheets - trim them to size if needed. Repeat with the ragu, bechamel and lasagne sheets, finishing with a layer of lasagne and the final fifth of bechamel. Spread it over the top evenly, then grate over the remaining parmesan. Bake for 35 minutes or until the top is well coloured and the sauces are bubbling. Turn around half way through baking for a more even crust. Stand for 45 minutes before serving warm rather than piping hot.