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.