Step 1) Score rind and pierce meat all over with a sharp knife or fork. Place in a large bowl or resealable plastic bag.
Step 2) Blend garlic, oregano, cumin, salt and pepper in a small bowl to make a paste. Stir in orange juice, oil and vinegar until blended. Pour over meat. Add onions.
Step 3) Marinate in refrigerator at least 8 hours, turning occasionally.
Step 4) Place meat in roasting pan; cover with onions, pour marinade over the top.
Step 5) Roast in 325°F oven, basting frequently with juices, about 3 to 3-1/2 hours or until meat thermometer reads 165° to 170°F. Cover loosely with foil; let stand for 10 minutes before slicing. Serve with onions and pan juices, if desired.