Preheat the oven to 160ºC . Rub the beef thoroughly with the onion, garlic, seasonings and a little vegetable oil. Wrap the meat in heavy-duty foil and seal the edges all around with tight crimps. Place the meat in a baking dish and bake for about 2.5 to 3 hours, or until the beef is tender enough to fall apart. If you hear much sizzling as the meat cooks, turn the heat down. When the beef is fork-tender, unwrap the top of the foil package, push aside the onions, and place the meat close to a broiler flame. Allow to broil only until the top surface begins to look dry and is starting to brown. Remove from the broiler and let the meat cool. Shred it into long shreds with your fingers, then store them in the refrigerator (for up to a week) or freeze for use as a fill.