Preheat oven to 325 degrees F. Cut small slits in the lamb and insert garlic pieces into the slits.
Combine olive oil, mustard, salt, pepper, rosemary and thyme and rub thoroughly over and into the meat. Place the lamb on a rack in a roasting pan and place in oven uncovered. Roast for 20 to 25 minutes per pound (approximately 1 3/4 hours).
Remove from the oven when meat thermometer registers between 130 degrees to 135 degrees F. Allow to stand 15 minutes before carving to allow juices to set and meat to continue cooking to about 145 to 150 F. for medium-rare. (Take care to never cook lamb until well done as it will be very dry.)
Remove kitchen string and slice into 1/2 thick slices against the grain of the meat. Serve with Cranberry Gravy if desired.