Heat the olive oil in a large Dutch oven or heavy-bottomed pot over medium heat. Add the chopped pancetta and cook until crispy and the fat has rendered, about 5 minutes.

Add the diced onion, carrot, celery, and minced garlic to the pot. Sauté the vegetables in the pancetta fat until softened and translucent, which should take about 6 to 8 minutes.

Increase the heat to medium-high and add the ground beef and ground pork. Cook the meat, breaking it up with a wooden spoon, until it is thoroughly browned and no pink remains, about 8 to 10 minutes.

Pour in the dry red wine to deglaze the pot, scraping up any browned bits stuck to the bottom. Let the wine simmer and reduce by half, which will take about 5 minutes.

Stir in the tomato paste, crushed San Marzano tomatoes, and whole milk. Season with a pinch of salt and black pepper, then bring the mixture to a gentle boil.

Reduce the heat to low, cover the pot partially, and let the sauce simmer gently for 1.5 to 2 hours. Stir occasionally, adding a splash of water if the sauce becomes too thick.

During the last 15 minutes of simmering, cook the spaghetti in a large pot of salted boiling water according to the package directions until al dente. Reserve 1/2 cup of pasta water before draining.

Toss the drained spaghetti directly into the Bolognese sauce, adding a splash of the reserved pasta water to help the sauce cling to the noodles. Divide among bowls, top with freshly grated Parmesan cheese, and serve immediately.