On December 22nd, coarsely chop onion, carrot and celery.
Heat olive oil in large heavy pot over medium heat until it starts to shimmer then add ground beef, ground pork, pancetta and vegetables. Cook, breaking up ground meat with a spoon, until moisture is evaporated and meat is well browned, about 30 minutes, and season with kosher salt and pepper.
Add wine to pot and bring to a boil, scraping browned bits from bottom. Slowly add milk, bring to boil, reduce heat to simmer until moisture is again almost completely evaporated, about 10 minutes. Add tomatoes and 2 cups of broth, bring to boil, reduce heat and simmer, adding water by the 1/2-cupfuls if sauce begins to look dry, and until flavors meld and sauce thickens, which will take nearly 3 hours.
Let sauce come to room temperature, cover and chill in the refrigerator at least 12 hours, and up to 2 days. As it sits, the sauce will become richer with flavor.