Season the meat with salt. Add the oil and the guanciale to a Dutch oven and cook until the guanciale has rendered its fat and begins to turn golden. Add the meat and brown it well on both sides.
When the meat is brown, remove it from the pan, then add the onion, celery, and carrot, and cook until onion is soft. Add the white wine and bring to a boil. Cook briskly until the liquid is reduced.
Nestle the meat into the vegetables and add the chopped tomatoes, passata, and season with salt and pepper. The meat might not be covered but that’s okay.
Cover and cook over low heat for about 4 hours, turning the meat once, and adding water if it gets too dry. (I did not need to add any water.)
At the end of cooking, stir in the grated dark chocolate to blend the flavors.
Serve over creamy mashed potatoes.