Dice the bacon or pancetta.

Chop the yellow onion.

Finely chop the carrots.

Finely chop the celery.

In a medium saucepan over medium-high heat, cook the bacon until it is crisp and has released almost all of its fat, about 6 minutes.

This is called rendering.

Add the onion, carrots, celery, salt, and pepper and cook, stirring frequently, until the vegetables are very soft and lightly browned around the edges, about 6 minutes.

Add the ground chuck and cook, stirring to break up any clumps, until the meat is browned, about 5 minutes.

Add the vinegar, garlic, and tomato paste and cook for 2 minutes, stirring constantly.

Add the beef broth and bring to a boil.

Lower the heat so that the sauce just simmers.

Simmer, partially covered, for 15 minutes.

Combine the milk and cream in a small mixing bowl.

After the sauce has simmered for 15 minutes, and at even intervals, start adding the milk-cream mixture little by little--1/4 cup or so at a time over 1 1/2 hours.

By the end of the 1 1/2 hours, the milk mixture should be completely incorporated and the sauce should be very thick and creamy.

This sauce is not supposed to be very "saucy"--it should be tender morsels of meat coated by a thick, creamy sauce.

Transfer the cooked pasta to a large heatproof bowl and add the sauce.

Toss the pasta with the sauce and serve immediately, garnished with grated Parmesan cheese, if desired.