Blend the tomatoes, ginger and garlic until smooth. Heat the oil in a large saucepan. Brown the lamb shanks in the oil for two or three minutes, getting a little colour on all sides. Remove and set aside. Reheat the oil, add the whole spices and bay leaves and, after 20 seconds, the onion; cook until the onion is well browned. Add the lamb, blended tomatoes, cumin, chilli powder, ground coriander, green chillies and salt; bring to a boil. Cover and cook over a low heat, stirring occasionally, until you have a little less than half the liquid you started with; it should take 15–20 minutes. Now increase the heat and brown the meat and sauce, stirring very often, until the oil comes out of the sauce and it has reduced considerably. This really intensifies the flavours. Add enough water to just cover the lamb shanks, bring to a boil, cover and cook over a gentle flame for 1–1¼ hours, or until the lamb is tender and done to your liking. Stir every 10–15 minutes and turn the meat in the sauce. Add a splash of hot water if the pan looks like it is running dry. When the lamb is tender and coming off the bone, reduce any excess liquid in the pan over a high heat until you have a creamy sauce. Taste and adjust the seasoning, stir in the garam masala and herbs and serve.