Cut off excess fat from lamb and slice 16 pockets into surface. Mix rosemary and garlic and stuffed mixture into each pocket. Massage the extra mixture onto the surface of lamb and season wit salt and pepper. Move lamb to a broiler rack or roasting pan.
Bake 1 1/2 hours or until a meat thermometer reads at least 140 degrees F. Let rest 15 minutes before slicing. Take lamb out of pan, saving 2 tablespoons of juice for the sauce.
Spoon saved drippings into a small resealable plastic bag. Seal bag; cut off 1 corner of bag. Drain liquid into a medium saucepan, stopping before the fat reaches the opening; throw away fat. Mix broth and rosemary sprig into drippings in pan and let boil. Lower heat, and simmer 1 minute. Whisk together port and cornstarch in a small bowl; pour into broth mixture. Let boil; cook 1 minute, stirring continually. Take out and throw away rosemary sprig. Spoon sauce over lamb.