First defrost the fish if you're using frozen. Then slice up into more-or-less bite size pieces, stick it in a container and rub the turmeric and a pinch of sea salt on it. Set it aside for about an hour or so in the fridge.
Prepare all the spices, onions, tomatoes in advance and it makes a mess of the counter, but easier to cook - as everything goes quite quickly...
Drop half a tablespoon of ghee in your pan and shallow fry the fish pieces. Set aside.
Add another half a tablespoon of ghee and stick your black mustard seeds in there and fry until the start to pop a bit. Not long.
Then add your onions and cook for about 3 or 4 minutes - stirring constantly.
When they are yellowish, add the chopped tomato, minced garlic, ginger, chill powder, and garam masala. Stir around to combine, fry for around a minute.
Pour in your can of coconut milk and let it all combine for a few more minutes.
Throw your cooked fish and the coriander (I used the preserved in oil kind, but fresh would be better - obviously!). Let it all simmer together for about 5 minutes.