1. Preheat the oven to 160°C/325°F/gas mark 3. Combine all of the ingredients for the marinade in a food processor and blitz until smooth. Put the lamb shanks in a large Pyrex bowl, pour over the marinade and toss to coat. Cover the bowl and leave to marinate for at least two hours or overnight if possible.
2. Heat the oil in a Pyrex cast iron casserole and fry the lamb shanks in batches for 5-6 minutes until golden brown all over. Remove from the casserole and set aside. Add the onions and fry for 6-8 minutes until softened and caramelised. Add the garlic and ginger and cook for a further 2 minutes. Add the spices, fry for two minutes until fragrant and then pour in the tomatoes and stock. Return the lamb to the casserole then bring to the boil. Cover with a lid then cook in the oven for 1 ½ hours. Remove the lid and cook for a further 30 minutes until the sauce has reduced and thickened.
3. Serve with steamed rice, naan bread and a sprinkle of chopped coriander.