Warm the oil in a heavy bottomed pot.
Add the chopped onion and saute until they are glossy.
Add the garlic, ginger, cummin seeds, curry leaves and curry powder and fry together gently for about 3 minutes, being careful not to burn the onions.
Add the chopped tomatoes and let this all simmer together for a few minutes until it becomes a lovely rich sauce.
Add the chopped lamb and about 1 teaspoon of salt. Turn the stove down to a very gentle simmer and let it cook for about 1 hour until the lamb is half cooked.
If the curry becomes dry during this hour, add up to 1 cup of cold water.
Add the potatoes and cook until they are soft.
Just before you are ready to serve add the chopped coriander and let the curry rest for about 10 minutes.