For the stew, heat the oil in a non-stick pan over a medium heat. Add the lamb and fry for 3-4 minutes, turning occasionally, until brown on all sides. Remove from the pan and set aside on a warm plate.
Add the mustard seeds to the same pan and fry for 30 seconds, or until the seeds start to pop. (CAUTION: keep the pan well away from your face and eyes.) Add the onion and fry for 3-4 minutes, or until golden-brown.
Meanwhile, in a food processor, blend the garlic and ginger to a paste, adding a splash of water to bind the mixture.
Add the garlic and ginger paste to the fried onions and continue to cook for 2-3 minutes, or until the water has evaporated from the paste and it has turned golden-brown.
Add the salt, sugar, ground coriander, ground cumin, ground turmeric and red chilli powder and cook for a further 20-30 seconds.
Add the puréed tomatoes and 100ml/3½fl oz of the water, stir well and bring the mixture to a gentle simmer. Simmer for 6-8 minutes, stirring well, or until the volume of liquid has completely reduced and the sauce has almost dried out.
Meanwhile, for the fenugreek dumplings, mix all of the fenugreek dumplings ingredients together in a bowl until well combined, then bring the mixture together to form a semi-soft dough.
Using your hands, roll the mixture into three sausage shapes, then pinch off six pieces from each sausage. Roll the pieces into smooth balls. Set aside.
When the sauce has reduced, add the coconut milk, lemon juice and the rest of the water. Continue to simmer for five minutes. Add the sweet potato to the stew and bring back to the boil. Reduce to a simmer and after six minutes, add the dumplings. Cover the pan with a lid and continue to simmer for 6-7 minutes, or until the potato is tender.
Add the reserved lamb and the chickpeas to the pan, cover again and cook for a further 4-5 minutes, or until the lamb is cooked to your liking.
Stir in the fresh coriander and garam masala just before serving.