In a large, wide Dutch oven or large, deep skillet, melt 4 tablespoons of the butter over low heat.
Stir in the onion, celery, carrot, and parsley and saute over low heat until the vegetables are quite soft but not at all browned, about 12 minutes.
Keeping the heat very low, add the ground meat and prosciutto. The meat must heat very gently, only enough to color it lightly on the outside; preventing it from hardening allows it to absorb the flavors of the other ingredients and to become delicate and creamy.
Stir in the salt and wine. Simmer very gently for several minutes until the alcohol evaporates and the liquid begins to be absorbed by the meat and vegetables.
Now add the milk and nutmeg. (It is important to add the milk before adding the tomatoes so that it will be absorbed by the meat.) Simmer gently for 10 minutes, then add the tomatoes and juice. As soon as the sauce begins to simmer, turn the heat down as low as possible.
Cover partially and continue to simmer, always over the lowest possible heat and stirring occasionally, for about 4 hours.
When the sauce is finished, stir in the remaining 1 tablespoon butter and the pepper. Taste and, if needed, add a bit more salt.
Add the pasta to the pot and toss with the sauce to incorporate it all together.
Serve in large bowls with a sprinkling of grated Parmesan.