How to Make It
Preheat the oven to 400°. In a bowl, blend together the scallions, lemon zest, and 1/4 cup of olive oil. Stir in 1 teaspoon of kosher salt and 2 teaspoons pepper.
Place the leg of lamb on a work surface, fat side down. With a boning knife, make a cut down the length of the leg bone that is just under the surface. Cut around the leg bone to make a pocket. Make 2 cuts on each side of the leg bone to separate the 2 muscles on each side. Spread out the part of the leg where the hip bone was. Rub the meat all over, on both sides with the lemon juice and season with salt. Let stand for 10 minutes. Set the leg fat side down and spread the scallion paste in the leg bone pocket, in between the 2 muscles on each side of the leg bone and all over the boned hip section. Fold the boned hip section into a neat bundle and set fat side up. Tie the leg, starting at the boned hip section, at 1 inch intervals. Place in a roasting pan, fat side up. Roast in the upper third of the oven for 40 minutes.
Turn the oven down to 325° and continue to roast for about 2 hours, until an instant read thermometer registers 160° when inserted into the thickest part of the leg.
Meanwhile, put the anchos in a shallow, heatproof bowl and cover with the hot water. Place a small bowl or ramekin on the anchos to keep them submerged. Let soak until softened, about 20 minutes. Reserve 1/2 cup of the soaking liquid. Discard the stems and seeds and coarsely chop the anchos.
When the lamb is done, transfer it to a carving board to rest for at least 10 minutes or up to 20 minutes. Pour the pan juices into a glass measuring cup and set the roasting pan over 2 burners on high heat. Add the chicken stock to the pan and boil, scraping up the browned bits on the bottom, until reduced to 1 1/4 cups, about 3 minutes. Skim the fat from the pan juices and transfer, along with the reduced stock to a small saucepan. Put the chopped anchos and reserved soaking liquid in a blender and puree. Stir the puree into the small saucepan with the reduced stock and pan juices and bring to a simmer over moderate heat. Add the soy sauce and season with salt and pepper.
Cut the strings off the leg of lamb and carve into 1/2 inch thick slices. Serve with the ancho sauce.