Heat olive oil in a large Dutch oven or heavy-bottomed pot over medium heat. Add the diced pancetta and cook, stirring occasionally, until it becomes crispy and has rendered its fat, about 6-8 minutes. Use a slotted spoon to remove the pancetta and set it aside, leaving the rendered fat in the pot.

Add the finely chopped onion, carrots, and celery (the soffritto) to the pot. Sauté, stirring frequently, until the vegetables are very soft and fragrant, about 10-12 minutes. Do not let them brown. Add the minced garlic and cook for another minute until aromatic.

Increase the heat to medium-high. Add the ground beef and ground pork to the pot. Use a wooden spoon to break up the meat and cook until it is thoroughly browned, about 8-10 minutes. Drain off any excess fat if necessary.

Return the cooked pancetta to the pot. Season the meat mixture with kosher salt, black pepper, and freshly grated nutmeg. Stir to combine.

Pour in the dry white wine to deglaze the pot, scraping up any browned bits from the bottom. Let it simmer and cook until the wine has almost completely evaporated, about 3-4 minutes.

Reduce the heat to medium and pour in the whole milk. Bring to a simmer and cook, stirring occasionally, until the milk is fully absorbed by the meat and has nearly evaporated. This crucial step creates a tender, velvety texture.

Stir in the crushed tomatoes, beef broth, and the bay leaf. Bring the mixture to a gentle simmer.

Once simmering, reduce the heat to the lowest possible setting. Cover the pot, leaving the lid slightly ajar to allow steam to escape. Let the sauce simmer slowly for a minimum of 2 hours, and up to 3 hours for the best flavor. Stir every 30 minutes to prevent sticking. The final sauce should be thick, rich, and deeply aromatic. Remove and discard the bay leaf.

About 20 minutes before the sauce is finished, bring a large pot of heavily salted water to a rolling boil. Add the tagliatelle and cook according to package directions until al dente.

Before draining the pasta, reserve about 1 cup of the starchy pasta water. Drain the pasta well.

Add the drained pasta directly to the pot with the Bolognese sauce. Add a splash (about 1/4 cup) of the reserved pasta water and toss everything together over low heat for 1 minute. This helps the sauce cling perfectly to every strand of pasta.

Serve immediately in warm bowls, topped with a generous amount of freshly grated Parmigiano-Reggiano cheese.