Ask a butcher to remove all bones, except shank, and to clean meat of all sinew and excess fat inside and out. Butterfly leg for stuffing.
Preheat oven to 375 degrees. Lay lamb flat on counter. Using a sharp knife, make several slits, about 2 inches long by 1/4 inch deep, so seasonings can penetrate meat. Sprinkle generously with salt and pepper. Puree whole garlic cloves with a little olive oil in a food processor. Add canned chipotles and blend into a paste. Spread the paste on the lamb and cover with the oregano. Roll meat to enclose stuffing and tie with string at 1inch intervals, keeping the diameter of the roll the same so that the roast cooks evenly. Shank bone will protrude at one end. Generously season outside with salt and pepper.
Heat oil in a large skillet over high heat. Brown lamb on all sides. Roast on rack in roasting pan 30 to 40 minutes for medium rare, 1 hour for well done. (A meat thermometer inserted to the center should read 160 to 165 degrees or a thin blade knife inserted to the center for 20 seconds should feel lukewarm on your lips.) Let sit at room temperature about 10 minutes before carving, so juices can run to center. Remove strings and slice meat thinly across the roll. Top with Pimento Sauce.
By removing the center bone, leg of lamb is much easier to carve and serve to guests.
Heat oil in a medium saucepan over moderate heat. Cook shallots and mushrooms until golden. Add wine, turn heat to high, and reduce by half. Add stock and reduce again by a quarter. Whisk in tomato paste, pimento puree and garlic. Break butter into small pieces and whisk into sauce until smooth. Adjust seasonings and strain through a fine sieve.
c.1997, M.S. Milliken & S. Feniger, all rights reserved