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.