Place the onion, celery and carrot in a food processor and blitz for about 10 seconds until finely chopped. Heat a deep-sided non-stick pan and then add the olive oil. Tip in the vegetables and pancetta or bacon and sauté over a medium heat for 1-2 minutes, stirring occasionally with a wooden spoon.
Increase the heat to high, then add the beef, stirring until well combined and cook for another few minutes until the meat is sealed, stirring continuously. Pour in the wine and simmer for 2-3 minutes until well reduced. Add the bouquet garni, a teaspoon each of salt and pepper and the tomatoes and chilli flakes, if using, and cook for another 2 minutes.
Add a little of the stock, cover and cook for 1.5 hours on a low heat, stirring occasionally and adding the stock a little at a time until it is finished and until the sauce has reduced and thickened. Season to taste.
Plunge the pappardelle into a large pan of boiling salted water and cook for 5-7 minutes until just tender or ‘al dente’. Drain and refresh under cold running water, then add to the sauce. Mix well, discard the bouquet garni and stir in the pecorino or Parmesan. Divide among hot plates and serve at once.