The day before you plan to serve this dish, trim the excess fat off the pork leg; make shallow slits all over the pork, using the tip of a knife; mash the garlic, salt, oregano, cumin, pepper, bay leaf and olive oil to a paste in a mortar; rub this mixture all over the roast, forcing it into the slits; combine the sour orange juice, sherry, and onions in a small bowl; place the roast in a large, heavy plastic bag; add the sour OJ mixture, making sure that the whole roast gets covered with the mixture; refrigerate and marinate the roast in the bag overnight, turning occasionally during the time.
Preheat the oven to 350 degrees F.
Drain the roast, and pat dry, reserving the marinade; place it in a lightly oiled, nonreactive, heavy roasting pan; cook the roast for 1 hour, turning once or twice to brown it on all sides.
Reduce the heat to 325 degrees; pour the marinade and onions over the pork; tent the pan with heavy-duty aluminum foil; (tear off a piece that is 1 1/2 times the width of your pan; fold it in half and crease the top; open it up, and place over the pan like a tent, tightly crimping edges to seal) continue roasting the pork, basting from time to time with the pan juices, until almost cooked, about 1 hour; add a little water or sherry if the pan dries out.
Uncover the roast and continue cooking until the internal temp reads*at least* 150 degrees F on a meat thermometer, about 30 minutes more; (most Cubans like the meat well done, so after sitting, it should be up to 180 degrees--it's safer, also).
Let the roast stand for 10 minutes before carving.
Note: this is traditionally served with black bean soup and white rice, and fried, sweet plantains; for dessert, serve a rich flan and you're set!