Preheat the oven to 320°F (160°C). In a large Dutch oven or other heavy, oven-safe pot, spread the sliced onions in an even layer, then scatter the dried fruits over the top. In a small bowl, whisk together the oil, cinnamon, ginger, nutmeg, turmeric, and salt. Rub the mixture all over the lamb.
Set the lamb on top of the onion and fruit mixture. Pour the water into the pot, around the edges (not directly over the meat), and place the rosemary sprig on top. Cover the pot tightly—use a sheet of parchment paper under the lid to create a better seal, if needed.
Transfer to the oven and roast for 4 hours, until the meat is very tender and infused with the flavors of the fruit and spices. About halfway through, carefully turn the roast and baste with the juices in the pot to keep it moist and flavorful.
Just before serving, for a festive finishing touch, top the roast with nuts tossed in sugar and cinnamon, then lightly toasted in a dry pan or in the oven until caramelized.