Position a rack in the lower third of the oven and preheat to 500 degrees F. Using a paring knife, make deep cuts, 2 inches apart, all over the lamb; rub with 2 tablespoons salt and several grinds of pepper. Transfer the lamb to a roasting pan, fat-side up. Squeeze the juice of 1 lemon on top.
Roast the lamb until most of the fat is rendered and the skin starts to brown, about 30 minutes. Transfer to a cutting board and let rest 30 to 40 minutes. Discard the fat from the pan and set the pan aside.
Meanwhile, trim the greens from the turnips. Cut any large turnips in half.
Roughly chop 2 bunches scallions and transfer to a food processor. Add the parsley, dill, marjoram, celery and garlic and pulse to form a coarse paste. Holding the bone, stand the leg of lamb up and rub the paste all over it; return to the roasting pan.
Cut the remaining 4 bunches scallions into 2-inch pieces. Scatter the scallions, turnips and tomatoes around the lamb and season with salt and pepper. Squeeze the juice of the remaining 2 lemons over the lamb and drizzle with the olive oil. Loosely cover with foil.
Reduce the oven temperature to 350 degrees F and roast the lamb 1 hour. Uncover and continue roasting until a thermometer inserted into the thickest part registers 145 degrees F for medium, 30 to 45 more minutes. Let rest 15 minutes, then slice. Serve with the vegetables and pan juices.