In a large dutch oven, heat the olive oil over medium heat. Add the pancetta and cook until fat starts to render, about 2-3 minutes. Add the carrots, celery, onion and garlic. Cook, partially covered, for 7-8 minutes to soften the vegetables.
Add the beef, veal (or pork), bay leaf, nutmeg, 1 tsp salt and 1/2 tsp pepper and cook the meat through, breaking it in to crumbles. Cook until no longer pink but do not brown it.
Add the wine and simmer until wine is absorbed by the meat. Stir in the tomato paste until incorporated, then add the chicken stock and cheese rind. Stir to blend. Partially cover with offset lid covering about 3/4 of the pot, and simmer on very low heat for 1 hour and 15 minutes, stirring occassionaly. You should still see some bubbles while simmering but not alot. (If sauce gets too thick, add a bit more chicken stock while simmering).
Remove the lid, add the milk and gently simmer for 40-45 more minutes, stirring occasionally. Remove and discard the bay leaf and cheese rind.
Cook the pasta in salted water until al-dente. Drain and reserve 1/2 C cooking water. Toss the pasta with the butter.
If the sauce is too thick, add some reserved pasta water. Taste the sauce and then season with more salt and pepper. (You will need to add more salt than you think to bring out the flavor, at least 1 tsp if not more).
Pour most of the bolgnese sauce (not all) into the desired amount of pasta and mix together before serving, to allow the pasta to absorb some of the sauce. Spoon into serving bowls, and top with a bit more bolognese, then top with grated parmesan.
Enjoy!