Heat a wide heavy sauté pan over medium-high heat. Add the oil to coat the pan, heating until shimmering and almost smoking. Add the salt pork and cook, stirring, until it is browned and crispy and has rendered much of its fat, about 6 minutes. Stir in the onions, carrots and celery and cook, stirring with a wooden spoon, until softened but not browned, about 6 minutes. Add the ground beef and cook, using the spoon to break up the meat, until it is cooked through, about 6 minutes. Stir in the tomato paste, stirring to coat the other ingredients, and cook until it turns a shade darker, about 2 minutes.
Pour in the wine and bring to a boil, stirring to loosen any flavorful bits cooked onto the bottom of the pan. Boil until the wine has evaporated, 6 to 7 minutes. Scatter the flour over the contents of the pan, stir it in, and cook, stirring constantly, for 1 minute. Stir in the stock, season with salt and pepper, and bring to a simmer. Stir in the milk and nutmeg and bring to a simmer, then lower the heat and simmer, covered, until the Bolognese is thick and aromatic, about 2 hours, or a bit longer if you prefer a denser sauce. During that time, periodically remove the lid, skim any fat with a tablespoon, season with a modicum of salt and pepper, and stir; continue to season periodically just until the sauce is assertively seasoned but not overly salty or spicy.
Meanwhile, toward the end of the sauce’s cooking time, fill a large pot about 2/3 full with water, salt it liberally, and bring to a boil over high heat. Add the pasta and cook until al dente, 2 to 3 minutes for fresh pasta.
Drain the pasta and add to the Bolognese. Add the butter and toss well, adding a little more stock if necessary, until the sauce thickly coats the pasta.
Divide among individual plates, grate some cheese over each service, and serve.