In a food processor combine the 1/2 cup olives, the onion, oil, anchovies, rosemary, and garlic. Cover and process with several on/off turns until a chunky paste forms.
Trim fat from meat. Cut 1/2-inch-wide slits about 1 inch deep all over lamb. Drizzle lemon juice over meat and into slits. Season with salt and pepper. Spread garlic-anchovy mixture over surface of lamb and into meat. Loosely cover lamb. Chill for 1 hour.
Preheat oven to 475 degrees F. Place lamb, fat side up, in a shallow roasting pan. Insert an oven-going meat thermometer into center of roast. The thermometer should not touch the bone. Roast lamb for 20 minutes. Reduce oven temperature to 325 degrees F. Roast lamb for 1 hour more.
Remove roast from oven. Add tomatoes, wine, and the 2/3 cup olives to roasting pan. Loosely tent lamb with foil. Roast for 1 to 1-1/4 hours more or until thermometer registers 135 degrees F for medium-rare doneness. Remove roast from roasting pan and transfer to a serving platter. Cover with foil and let stand for 15 minutes. Temperature of the meat after standing should be 145 degrees F. (For medium doneness, roast 2-1/4 to 2-3/4 hours or until meat thermometer registers 150 degrees F before removing from oven. Temperature of the meat after standing should be 160 degrees F.)
For sauce, place roasting pan over two burners; bring mixture to boiling. Reduce heat. Simmer, uncovered, for 15 to 20 minutes or until desired consistency, stirring occasionally. Serve sauce with lamb. If desired, serve with polenta.