Heat the oven to 250ºF. Place a 12-inch cast-iron skillet over high heat for 2 minutes.
Meanwhile, make a big ole pouch with a double layer of heavy-duty aluminum foil and set aside.
Rub both sides of the meat with the salt and cumin, and brown on both sides until golden brown, 2 to 3 minutes. (Flip with tongs, not a fork. You don’t want to punch any holes in the meat.) Transfer the roast to the foil pouch.
Lower the heat to medium and add enough oil to coat the bottom of the pan, followed by the onions and garlic. Stir constantly until the garlic is browned slightly, 2 to 3 minutes.
Add the tomato juice, vinegar, olives, and raisins, and bring to a boil; cook until the liquid is reduced by half, 3 to 5 minutes. Lift the roast with tongs and place half of the reduced mixture under the roast, then top with the remaining mixture. Close the pouch, and wrap tightly in another complete layer of foil. Triple-fold your seams so that the package is water-tight.
Place on the middle rack of the oven and cook for 3 1/2 hours, or until a fork pushes easily into the meat. (Just in case the pouch does spring a leak, place a sheet pan on the lower rack.) Remove from the oven and set the pouch aside, unopened, for at least 30 minutes. Serve with the braising liquid.