For the stew, heat the oil in a nonstick pan over a medium heat. Add the lamb and fry for 3 to 4 minutes, turning occasionally, until the lamb is brown on all sides. Remove the lamb 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. Be careful of the popping seeds and splattering oil. Add the onion, and fry for 3 to 4 minutes, or until golden brown. Meanwhile, in a food processor, blend the garlic, and the 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 to 3 minutes, or until the water has evaporated from the paste and it has turned golden brown. Add the salt to taste, sugar, ground coriander, ground cumin, ground turmeric and red chile powder and cook for a further 20 to 30 seconds.
Add the pureed tomatoes and 3 1/2 ounces of the water, stir well and bring the mixture to a gentle simmer. Simmer for 6 to 8 minutes, stirring well, or until the volume of liquid has completely reduced and the sauce has almost dried out.
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 dough into 3 sausage shapes, then pinch off 6 pieces from each sausage. Roll the pieces into smooth balls, and set aside.
When the sauce has reduced, add the remaining water, coconut milk, and lemon juice. Continue to simmer the stew for 5 minutes. Add the sweet potato to the stew, and bring it back to a boil. Reduce to a simmer and after 6 minutes, add the dumplings. Cover the pan with a lid and continue to simmer for 6 to 7 minutes, or until the potato is tender.
Add the reserved lamb and the chickpeas to the pan, cover again and cook for another 4 to 5 minutes, or until the lamb is cooked to the desired degree of doneness.
Stir the fresh cilantro and garam masala into the stew, just before serving.
For full-flavored gravy with depth, make sure the onions are well browned. To ensure the dumplings are light, don't overwork them. The baking powder helps make super light dumplings.