To make the sauce: Add a tablespoon of oil to a thick bottomed pan over a medium heat. Add the onion, garlic and ginger and cook gently for about 15 minutes until the onion softens. Then add the mustard seeds and cook for another minute or two until they begin to crackle.
Add all the spice mix ingredients to the pan and stir them into the mixture. Cook for a couple of minutes then add the chopped tomatoes, coconut milk and curry leaves.
Bring to a simmer and cook for about 10 minutes. Add the cubes of fish and simmer very gently for 3-5 minutes. Then remove from the heat. Taste and season.
To serve: Remove the curry leaves and sprinkle with chopped coriander. Serve immediately with rice or noodles and a salad.