Heat oven to 160C/140C fan/gas 3. Heat 1 tbsp oil in a heavy-bottomed casserole, season the shanks, then brown them 2 at a time on all sides, adding the remaining oil for the second batch. Remove the lamb and add the onions. Fry them quite briskly, about 30 secs, add the ginger, garlic and chopped chilli, then turn the heat down and cook for 1 min. Add 1 tbsp sugar, stir, then add the star anise, lemongrass, stock, purée and seasoning. Bring to the boil.