Tip the yogurt, cumin, turmeric, 1 tsp sea salt and the lamb shanks into a large mixing bowl, then mix to coat the lamb. Cover and pop in the fridge for a couple of hrs, or overnight if you have time.
Heat oven to 160C/140C fan/gas 4. Heat the oil in a large flameproof casserole dish over a medium heat, add the shanks and brown all over for 10 mins, then remove from the dish. Scatter the onions into the dish and fry for 10 mins until golden brown. Stir in the curry powder, garlic and ginger and cook for 3 mins until aromatic. Add the lamb shanks back to the dish along with the tomatoes, chillies, curry leaves, cardamom pods and lime pickle. Give everything a good stir and pour over the stock. Bring up to a simmer, cover, then transfer to the oven and cook for 3 hrs.
Remove the lid and cook for 1 hr more – this will help reduce some of the liquid and char any exposed meat. When the lamb is very tender, leave to rest for 30 mins or leave to cool completely and reheat the next day for the best flavour. (Can be made up to two days in advance.) Scatter with chopped mint and serve with naan bread and rice on the side.