Step 1 Heat the oven to 350 degrees. Lay the lamb flat on a work surface opened up like a book with the meat side up. The meat may be thick in a few parts; slice through the thick parts to allow the meat to lay flatter. Be careful not to cut all the way through the meat; leave at least one-half inch of meat at the bottom so you can roll the meat later. Cover the cut side of the lamb with plastic wrap. Lightly pound with a meat mallet or a rolling pin a few times to slightly even the meat. Season both sides with about 1 teaspoon salt and three-fourths teaspoon pepper, or to taste, and set aside.
Step 2 Make the stuffing: In a large pot, heat 2 tablespoons of olive oil over medium heat. Add the onion and cook until it is soft, about 5 minutes. Add the garlic and cook until it is fragrant, about 3 minutes. While the onions and garlic are cooking, sort the greens, discarding any tough stems, and chop them. Place them in a bowl and cover with water. When the garlic is fragrant, lift the greens from the bowl by the handful and transfer to the pot, lightly shaking off a little of the water. As the greens wilt and reduce, add another handful. Add the red pepper flakes, lemon zest and one-half teaspoon salt and cook until the greens are tender and there is almost no water left in the pot, about 10 minutes.
Step 3Season with a generous grinding of black pepper and more salt if necessary (some feta cheeses are very salty; some are milder). Set the greens aside to cool slightly. You should have about 2 cups. Fold the feta and the pine nuts into the cooked greens. There should be only small pieces of the feta visible.
Step 4Stuff the lamb: Pat dry the cut side of the lamb with a paper towel. Have butcher's twine handy. Spread the greens mixture over the lamb in as close to an even layer as you can manage. Leave about one-half inch of meat exposed around the border. You may not use all of the stuffing. The meat will be roughly triangular. Starting with the narrow end, roll the lamb fairly tightly into a cylinder, as you would a jellyroll. Tie a length of butcher's twine around the middle to keep it closed. Tie another length around the lamb end-to-end, folding the flaps of meat over the ends to close them and keep the filling from leaking out. Repeat with a new loop of twine every inch or two. Tuck the rosemary sprigs into the roast at regular intervals under the twine.
Step 5Rub the exterior of the lamb with a little olive oil and place it on a rack in a roasting pan. Place in the oven and roast 45 minutes, then turn the lamb over so it will brown evenly. Continue to roast to an internal temperature of 135 degrees, about 1 1/2 to 2 hours total. Be sure to check the temperature in several places to make sure you're not hitting a pocket of stuffing, which will give you a false reading.
Step 6When the lamb is done, remove it to a cutting board and tent it with foil to keep warm. It should rest for about 30 minutes befo