Preheat your oven to 350F
Pat the roast dry using paper towels. With a paring knife, score the skin or fat to make a diamond-shaped pattern. (Make sure not to cut the underlying meat)
Season the roast liberally with salt, making sure it’s salted all over.
Place garlic, onion (if using), oregano, cumin, whole black pepper, orange juice and vinegar in a blender. Process until it has a very smooth texture. Set aside.
Turn the roast on the meat side (the one that has no skin on), and with the pairing knife make some incisions about ½ inch deep, about 1 inch apart from each other.
Place roast in the roasting pan, pour the marinade over it, and cover with aluminum foil. If using a baking bag, place roast with the marinade inside the bag that has been previously treated with a little flour, with the incisions that the manufacturer indicates in the package.
Put the roast in your preheated oven, and baste with the marinade, as well as the cooking juices that will be released into the pan during the roasting time. If possible, turn the roast once. Roast 15 minutes per pound, or until the meat thermometer, inserted into the thickest part of the meat, reads 165 degrees.
Remove the roast from the oven; let it rest for at least 15 minutes before carving. Pour the juices from the roasting pan into a gravy bowl or sauce bowl; season with salt if needed.