1. In a medium enameled cast-iron casserole, melt the butter in the oil. Add the garlic, pancetta, carrot, onion and celery and cook over moderately low heat until the onion is golden, about 5 minutes. Add the ground beef and pork and cook over moderate heat, breaking up the meat with a wooden spoon, until no pink remains, about 8 minutes. Stir in the mortadella and tomato paste and cook for 2 minutes. Add the wine and cook, stirring, until reduced by half, about 3 minutes. Add the tomatoes with their juices, the stock, nutmeg and 1 tablespoon each of the parsley and basil and bring to a boil. Season with salt and pepper and simmer over low heat, stirring occasionally, until very thick, about 1 1/2 hours. Keep warm.
2. In a large pot of boiling salted water, cook the spaghetti until al dente. Drain and return it to the pot.
3. Stir the cream and the remaining 2 tablespoons of parsley and 1 tablespoon of basil into the meat sauce. Season the sauce with salt and pepper, then add 2 cups to the spaghetti and toss. Transfer the spaghetti to a large bowl, top with the remaining sauce and serve. Pass the Parmesan cheese at the table.
Make Ahead: The Bolognese sauce can be refrigerated for up to 5 days.