Firstly, place the lamb shanks on a plate and rub with salt and oil. Leave to one side.
Next, put the ginger in a blender, add 5 tablespoons of water and process to a fine paste. Leave to one side.
Add the nutmeg, mace, fennel, bay leaves, cinnamon, ground ginger, black pepper, nigella seeds, cloves and cumin seeds to a bowl and set to one side.
Preheat the oven to 160C/140 fan. Put a large flameproof casserole dish/pan with a tight fitting lid (at least 30 cm wide) the hob, put on medium heat and add the butter. Once it has melted, add the onions and brown, this should take about 5 minutes. Then remove and set to one side.
Pour the oil in the pan, when hot, add the shanks and brown. Use tongs to turn them in the pan, at least two minutes each side. Then remove the shanks and set to one side.
Then turn the heat to low, and add the ginger paste. Stir continuously, make sure the oil is not too hot, otherwise it will be burn the ginger paste. Stir for 4-5 minutes until the ginger is light brown, then add the paprika and chilli powder and stir for a minute. Then add the remaining spices set aside and 1 teaspoon of salt. Mix well.
Then add the browned onions to the pan, and add 1.2 litres of warm water. Mix well then add the shanks back into the pan. Cover the pan with the lid and put in the middle of the oven.
Next, leave to cook the for 3 hours, turn the shanks and baste every 30 minutes. When they have cooked for 3 hours, remove the shanks and set aside in the serving bowl. Spoon off as much excess oil and put the pan back on the hob.
Finally, mix the chapatti flour with 4 tablespoons in a bowl, so it forms a paste. Then add the paste into the pan and stir into the curry sauce for 5-6 minutes so it thickens. Then pour the sauce into the serving bowl. Serve with naan and sliced ginger and sliced green chillies.