Place the onion, tomato, peppers, cumin and ½ cup (125ml) water in a slow cooker. Stir to combine.
Heat half the oil in a large frying pan over high heat. Cook the lamb, turning occasionally, for 5 mins or until brown all over. Combine the garlic, sugar and remaining oil in a small bowl. Season. Brush the lamb with garlic mixture. Transfer the lamb, fat-side down, to the slow cooker.
Cover and cook, turning the lamb halfway through cooking, for 4 hours on high (or 6 hours on low) or until the lamb is falling off the bone. Stir in the beans. Cover. Cook on high for 30 mins or until beans are tender.
Meanwhile, place the potato in a saucepan and cover with cold water. Bring to the boil over high heat. Reduce heat to medium. Cook for 15 mins or until the potato is tender. Drain. Set aside for 5 mins. Return potato to the pan and use a potato masher to mash until smooth. Add the milk, lemon rind, lemon juice and extra oil. Stir to combine. Place over low heat. Cook, stirring, for 2-3 mins or until heated through. Season.
Transfer the lamb mixture to a serving dish. Sprinkle lamb with dill and dukkah. Serve with the mash.