Which of these ingredients is key to making a Bolognese sauce?

Traditionally, bolognese sauce is made with a combination of meat (pork and beef, sometimes veal) and often contains cured pork, such as pancetta, to help season it. The meats are cooked in a heavy pot with softly cooked carrots, celery, and onions. There is a hint of warm spice from freshly grated nutmeg. Milk coats the meat and adds a mellow sweetness to the sauce.

Source: Martha Stewart