In a small dry skillet, toast the sesame seeds over moderate heat until golden, 45 seconds. Transfer the seeds to a plate.
In a large enameled cast-iron casserole, combine the oil, lamb, onion, cilantro, cinnamon stick, ginger, saffron, a pinch of salt and ½ teaspoon of pepper. Add the water and bring to a boil. Reduce the heat to moderately low and cook, skimming the stew a few times, until the lamb is tender, about 1 ½ hours.
Using a slotted spoon, remove the onion, cilantro and cinnamon stick and discard. Add the prunes to the casserole and simmer for 15 minutes. With a slotted spoon, transfer the lamb and prunes to a bowl. Boil the cooking liquid over high heat until slightly reduced, about 5 minutes. Add the honey and boil for 5 minutes. Return the lamb and prunes to the casserole and stir in the orange-flower water, if using. Season with salt. Transfer the stew to a bowl, sprinkle with the toasted sesame seeds and serve.
