Place Curry Paste ingredients in a tall jug or milkshake container then blitz on high for 5 to 10 seconds until the onion is pureed. Add more water if needed to make it puree.
Heat oil over medium heat in a large pot. Add black mustard seeds and let them sizzle for 30 seconds.
Add red onion and cook for 3 minutes until edges start to tinge with gold.
Add curry paste and cook for 3 minutes.
Turn heat up to medium high. Add tomato paste and tomato pulp, cook for 2 minutes.
Add water, coconut milk, sugar, salt and chilli powder if using. Stir, then bring to simmer.
Simmer for 2 minutes, add tomato and green chilli.
Simmer for another 3 minutes, stirring every now and then.
Add fish, stir, cook for 3 to 4 minutes until fish easily flakes.
Remove from stove and transfer to serving bowl. Garnish with coriander/cilantro and fresh green chillies if desired. Serve with basmati rice!
