Preparation

Make the stuffing

• Position a rack in the center of the oven and heat to 350°F. Arrange the bread in a single layer on a large rimmed baking sheet and toast until just crisp, 10 to 15 minutes. Cool to room temperature.
• Measure 2/3 cup of the orange juice into a small bowl. Add the currants and soak until soft, 25 to 30 minutes. Strain the currants over a bowl, pressing on them to extract more liquid. Reserve the liquid and currants separately.
• Heat the oil in a large skillet over medium heat. Add the onion and garlic, and cook, stirring occasionally and adjusting the heat as necessary, until very soft but not browned, about 8 minutes. Transfer to a large bowl. Add the bread, currants, zest, pistachios, mint, oregano, 1 tsp. salt, and 1/4 tsp. pepper. In a small bowl, whisk the broth, eggs, and reserved currant soaking liquid, and pour over the bread mixture. Toss well, let sit for 5 minutes, and toss again. Cover and chill for at least 30 minutes and up to 1 day.

Wrap the lamb

• Pat the lamb loins dry and season well with salt and pepper. Stack the loins one on top of the other.
• Lay a length of plastic wrap on a work surface so that it measures at least 20×20 inches; overlap a couple of pieces of plastic wrap as needed. Shingle the grape leaves (allow a good overlap) with the smooth sides down into a 14×14-inch square. (If the meat seems larger, add more leaves to accommodate.) Press the stuffing mixture onto the leaves, leaving a 1-1/2-inch border on the top and bottom and 1 inch on each side. Position the stacked lamb loins on the center of the stuffing. Lift the plastic to help wrap the grape leaves and stuffing around the lamb and continue to roll like a sushi roll, encasing it completely. Tuck in the ends of the grape leaves, then wrap the roll tightly in the plastic wrap, twisting the excess plastic on each end tightly to compress the roast. Tuck the twists under the roast to keep snug. (if it feels loose, wrap it tightly in another layer of plastic wrap.) Refrigerate for at least 1 hour and up to 1 day. Longer is better as it helps firm everything up.

Roast the lamb and make the sauce

• Position a rack in the center of the oven and heat to 375°F. Line a roasting pan with parchment (for easy cleanup) and set a rack in it.
• Remove the plastic wrap from the lamb, and pat the leaves dry with paper towels. Tie the roast with butcher’s twine in about 5 even intervals. Place the roast on the rack and brush the grape leaves with olive oil.
• Roast to medium rare (120°F), 45 to 60 minutes (begin checking early). Let rest for 15 minutes.
• Just before serving, combine the mint, parsley, garlic, lemon zest, and juice. Whisk in the oil to make a thick but pourable sauce. Season to taste with salt.
• Slice the roast with a very sharp knife (serrated is best here) into thick slices, removing the twine as you go. Serve topped with the herb sauce.