Heat the oil in a large pan. Once hot, add the lamb, and cook until browned all over. Remove from the pan (leaving the oil) and set aside.
Add the onion, ginger and garlic to the oil, and cook over a low heat until the onion becomes translucent - don't let it brown.
Stir in the Massale spices, along with the potatoes, tomatoes and two thirds of the coriander. Cook for around 5 minutes, until the ingredients are well blended. Be sure to stir it regularly so it doesn't stick and burn.
Add the lamb back to the pan, along with enough hot water to make the amount of sauce you desire, plus the tomato purée. Stir well, cover and cook on a low heat for around 30 - 35 minutes, until the lamb is cooked and tender.
Sprinkle over the remaining fresh coriander to serve, and serve with basmati rice.
