Butterflying the lamb gives you options you don't have with a bone. A good butcher will be happy to do this for you. Here, I've made a very flavorful stuffing from sun-dried tomatoes that looks great when you carve.
Normally, I don't see the point in mincing herbs, but rosemary is hard to eat. If it's a flavoring agent, you can just pull the woody sprigs out at the end, but if you want to eat it — and lamb loves rosemary — it has to be very finely chopped.
In a food processor, combine all of the ingredients for the stuffing and puree for 45 to 60 seconds, to a smooth, thick paste. Reserve about 2 tablespoons of the stuffing.
Lay the lamb out on a work surface with the fattier side down. Season generously with pepper and spread the stuffing over it in an even layer, pressing down into the crevasses. Drizzle with a little olive oil and roll the lamb up in a spiral, seasoning the fatty side with salt and pepper as you roll. Tie in 3 or 4 places crosswise and 1 or 2 places lengthwise. (Tip: Twist the string around itself 3 times instead of just once before you pull it tight, so it won't loosen as soon as you let go). Ideally, allow to sit on a rack, uncovered in the refrigerator overnight, to dry the surface well and develop all the Greek flavors in the piece.
Preheat the oven to 375 degrees F. In a small roasting pan, whisk the reserved stuffing with the water, mustard and garlic puree. Add the rosemary sprigs and place a rack in the pan (the rack should not touch the liquid).
Again, season the lamb on all sides very generously with salt and pepper. In a large heavy skillet, warm the oil over medium-high heat. When it is very hot, sear the lamb well on all sides, using tongs and leaning the meat up against the sides of the pans to sear the thinner sides and cut ends. Transfer the lamb to the rack seam side up and roast for about 1 hour, basting every 15 minutes with the pan liquid. (When it's medium-rare at 140 degrees F, a skewer inserted at the thickest point of the meat should feel warm when pressed against your lower lip.)
Rest for about 15 minutes. Slice 1/4-inch thick pieces, drizzle with the pan sauce and finish with extra-virgin olive oil.
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