In a heated tagine (or pot with a lid), heat the olive oil and sear the lamb with the onion for 2 minutes. Add the sugar, cinnamon, saffron, and salt and pepper to the lamb. Let cook for another minute, stirring occasionally. Add the vegetable stock, cover, and let cook on low to medium heat for 45 minutes to 1 hour. Turn off the heat, add the prunes, and let rest with the lid on for 5 minutes. Sprinkle with the sesame seeds before serving.