Begin by peeling the onion and carrot. Chop the onion, carrot, and celery into small dice, one vegetable at a time. To achieve a finer texture, use a rocking motion with your knife to mince the chopped vegetables further. Slice and dice the pancetta into small strips or cubes. Heat a shallow casserole or skillet with a lid over medium-low heat. Add the olive oil, butter, vegetables, and pancetta, then cook gently, stirring frequently, until the vegetables are softened and lightly golden. Using your hands, crumble the ground beef and pork into the pan. Break up any remaining clumps with a wooden spoon and cook until all the liquid evaporates and the meat starts to sizzle and crackle in the fat—this signals that the bolognese is ready for the next step. Pour in the dry white wine and let it cook off completely. Stir in the beef stock, tomato paste, freshly grated nutmeg, salt, and black pepper. Lower the heat to very low, cover with the lid, and let the sauce simmer gently for 2 hours. Check occasionally to ensure the sauce is simmering, not boiling, and add a little water if it becomes too dry. Taste and adjust seasoning as needed. While the sauce simmers, bring a large pot of salted water to a boil. Cook the tagliatelle according to the package instructions, or for fresh homemade pasta, cook for 2 to 3 minutes. Drain the pasta well, reserving about one cup of the cooking water. Add the pasta to the sauce—if your pan is large enough—or combine both in the pasta pot. Toss gently but thoroughly, incorporating some Parmesan cheese for extra creaminess. Transfer to a serving dish and serve with additional Parmigiano-Reggiano cheese on the side.