Using a small knife, make several incisions in the lamb. Stuff a half clove of garlic and a few rosemary sprigs into each cut. Drizzle a small amount of olive oil over lamb. Place the meat in the slow cooker. Squeeze the lemon over the lamb, and drop the lemon shells into the slow cooker. Cook the lamb on low for 8 hours or until the meat easily pulls apart. It should register at least 135 degrees on a meat thermometer.