Heat the oil on a medium heat and stir fry the curry leaves (if using), mustard seeds and ginger for 30 seconds. Add the shallots and turn down heat to simmer gently for 7-8 minutes till soft and starting to darken.
Add the chilli power, coriander and turmeric and stir for a minute before adding the tomatoes, sugar and the tamarind and cook for another 5 minutes.
Add the stock and simmer on a medium heat for 10 minutes. Then add the coconut milk and the fish, and gently stir through and cook for another 10 minutes.
Stir through the lemon juice, season and serve with steamed basmati rice.