Preheat oven to 400 degrees. Cut small slits all over the lamb with the tip of a sharp paring knife, and insert the garlic and ginger slivers into them. Place the lamb in a shallow roasting pan and brush it all over with the olive oil. Combine the pepper and all of the herbs in a small bowl, and pat the mixture all over the lamb. Place the lamb in the oven and immediately reduce the temperature to 350 degrees. Roast for 1 1/2 hours or until the temperature registers 135 degrees for rare meat, 140 degrees for medium rare meat, when the thermometer is inserted into the thickest part of the leg. Let the lamb rest for about 10 minutes before carving. (the internal temperature will rise slightly as the lamb rests. If you prefer the lamb more well done, roast it for another 10 minutes or so.) Place the lamb on a serving platter and garnish with fresh rosemary sprigs. Cut into thin slices and serve.