Cut small slits all over meat, and insert garlic and olives. Place meat in a large bowl, and add mojo marinade. Cover and chill 1 to 2 days.
Remove meat from marinade; discard marinade. Pat pork dry, and rub Spice Blend all over pork.
Heat oil in a large ovenproof skillet or pan over medium-high heat until hot. Brown roast on all sides. Scatter sliced onion around meat in bottom of pan. Cover and bake at 325° for 5 hours or until pork is tender and meat falls off the bone. Serve with pan juices and onion, if desired.