Preheat the oven to 300°. Using a sharp knife, score the pork skin in a crosshatch pattern. Make deep slits all over the pork and insert a slice of garlic into each slit. Season the pork with salt and pepper.
In a large enameled cast-iron casserole, heat the olive oil. Add the pork and cook over moderate heat, turning occasionally, until browned all over, about 12 minutes.
Transfer the pork to a plate.
Add the shallots to the casserole and cook over low heat, stirring, until lightly browned, 5 minutes. Stir in the cumin, coriander and mustard. Add the vinegar and scrape up any browned bits from the bottom of the casserole. Add the wine, stock and sage and bring to a boil. Return the pork and any accumulated juices to the casserole. Cover and braise in the oven for 3 hours, or until the pork is very tender.
Carefully transfer the pork to a platter and cover with foil. Set the casserole over high heat and boil the braising liquid until it is slightly reduced and thickened, about 10 minutes. Add the grapes and boil until they just begin to soften, 8 minutes.
Remove the casserole from the heat and whisk in the butter, parsley and chives.
Slice the pork and serve with the vinegar sauce.