Heat half the oil in a large, heavy-based frying pan or casserole dish over a medium-high heat, and fry the mince and pancetta until golden. Transfer to a bowl using a slotted spoon and set aside.
Add the remaining oil to the pan and fry the onion, celery and carrot for 10 mins until just softened. Add the garlic and cook for 1 min more, then return the meat to the pan.Tip in the wine, bring to a simmer and cook until the mixture is reduced by half. Stir in the tomato purée, tomatoes, stock, bay, rosemary and sugar. Simmer, covered, for 30 mins, then remove the lid and simmer uncovered for another 10 mins until reduced. The ragu will keep in the freezer for up to two months. Leave to cool completely before freezing.
Meanwhile, make the cheese sauce. Melt the butter in a saucepan until foaming, then stir in the flour and cook for 2 mins. Stir in the mustard, then remove from the heat and gradually whisk in the milk in small additions. Return the pan to the heat and simmer for 5-6 mins, whisking continuously until thick and smooth. Add the nutmeg, parmesan and 80g of the cheddar, then season to taste.
Heat the oven to 200C/180C fan/gas 6, and cook the macaroni in a large pan of boiling water for 5 mins. Drain, then stir into the cheese sauce. Spread the ragu into the base of a large rectangular baking dish (ours was 28 x 22 x 5cm), then spoon over the mac ‘n’ cheese and gently spread out using the back of a spoon to cover the ragu. Top with the remaining cheddar and the mozzarella, and bake for 25-30 mins until golden and bubbling. Leave to rest for 5 mins, then scatter over a few basil leaves to serve, if you like.
