Place a heavy 24-26cm non-stick casserole pan over low heat. Add the pancetta and olive oil, then gently heat until the pancetta begins to melt and release its fat.
Finely chop the onion, celery, and carrot by hand with a knife. Add them to the pan with the pancetta and cook gently over a low heat, stirring often with a wooden spoon, until soft and fragrant but without letting them colour.
Increase the heat to medium and add the meat. Break it up with the spoon and cook for around 10 minutes, stirring regularly, until it starts to sizzle and take on some colour.
Pour in the wine and let it cook off completely over a medium heat.
Stir in the tomato paste and purée, then add about a cup of boiling stock or water. Cover and simmer gently for around 2 hours (or up to 3, depending on the meat), topping up with hot liquid as needed. If using milk, add it halfway through the cooking and let it reduce fully before continuing.
Season with salt and pepper to taste. The finished sauce should be deep maroon in colour, thick, and glossy.