Wrap the sage, thyme and rosemary in cheesecloth and tie with kitchen twine. Add this bundle to the casserole along with the onion, carrot, celery, leek and garlic and cook, stirring, until the onion is translucent, about 2 minutes. Stir in the wine, scraping up any browned bits on the bottom of the casserole and bring to a boil. Return the pancetta and pork shoulder to the casserole and simmer over moderately low heat until liquid is almost fully evaporated, about 10 minutes. Add the stock and tomato puree and return to a boil. Cover and simmer the sauce over moderately low heat for 1 hour, until the meat is very tender.