Step 1Spread the leg of lamb out on a cutting board so that the interior is facing up. With a sharp knife, remove any extra fat and tendons.
Step 2In a food processor, combine the garlic, salt, pepper, olives, figs, rosemary leaves and olive oil and pulse until coarsely chopped and combined. Spread the mixture over the inside of the lamb, cover with plastic wrap and refrigerate 2 hours.
Step 3Heat the oven to 425 degrees. Remove the lamb from the refrigerator and let stand 30 minutes. Reassemble the leg by rolling it back up with the fig-olive mixture inside. Tie the leg securely with kitchen twine, tying at 3- to 4-inch intervals crosswise as well as twice lengthwise. Stuff back in any of the fig-olive mixture that has fallen out of the rolled leg. Rub the outside of the lamb with just enough olive oil to coat and sprinkle with salt to taste. Place the rosemary sprigs on the bottom of the roasting pan and then put the trussed leg on top of the rosemary.
Step 4Roast the lamb for 15 minutes, then reduce the heat to 375 degrees. Continue to roast until a thermometer inserted into the thickest part of the meat reads 135 degrees for medium rare (or, if you prefer, 140-145 degrees for medium), checking after 1 hour, then every 15 minutes thereafter. Total cooking time will be about 1 hour for a 4-pound roast. Remove from the oven, tent with foil and allow to cool to room temperature. When cool, wrap tightly with plastic wrap and refrigerate overnight. Before serving, unwrap the lamb, remove string and cut the meat into thin slices.