Trim any discoloured spots, fat and membrane from the chicken livers. Wash the livers and then pat them dry with paper towels. Roughly chop the livers.
Heat the oil in a large frying pan over medium heat. Add the chicken livers, onion, garlic, and beef and pork mince. Cook, stirring frequently with a wooden spoon to break up the mince, until the meat is brown.
Add the tomatoes and juice, the stock and the tomato paste. Stir until well combined, breaking up the tomatoes with a wooden spoon. Add the bay leaves and bring the sauce to the boil. Reduce the heat to low, cover the frying pan with a lid and simmer on very low heat, for 1 hour, or until the sauce is thick and rich. Stir the sauce from time to time during cooking to prevent the meat from sticking.
Bring a large saucepan of salted water to the boil. Add the spaghetti and cook, uncovered, stirring from time to time, for 10–12 minutes, or until the pasta is al dente (Be careful not to overcook the pasta). Drain.
Remove and discard the bay leaves from the sauce. Stir in the oregano. Remove from the heat, season to taste with salt and pepper. Stir in the butter.
To serve, turn the cooked, drained pasta into a heated bowl and top with the sauce. Sprinkle with a little of the Parmesan cheese and pass the remaining cheese separately.
Many versions of traditional Bolognaise sauce are made with this combination of chicken livers, beef mince and pork mince. If you prefer to omit the chicken livers, however, you can double the amount of minced beef and pork.