Heat the oil and butter in a pan and gently fry the chopped onion. When the onion begins to soften add the pancetta or chicken livers and fry for a minute or two longer. Add all the meat and fry until golden brown. Pour in the wine and boil rapidly to evaporate. Then add the tomato purée and tomato pulp/passata. Season to taste, cover the pan and simmer for at least two hours stirring from time to time.
Cook the pasta in plenty of boiling salted water until al dente, then drain and mix with the sauce.