Place Curry Paste ingredients in a tall jug or milkshake container (something that fits the head of a blender stick) then blitz on high for 5 to 10 seconds until the onion is pureed. Add more water if needed to make it puree.

Sizzle black mustard seeds: Heat oil over medium heat in a large pot. Add black mustard seeds and let them sizzle for 30 seconds - careful, they might pop!

Saute onion: Add red onion and cook for 3 minutes until edges start to tinge with gold.

Cook off curry paste: Add curry paste and cook for 3 minutes - to evaporate water, make spices bloom and cook garlic & ginger.

Cook off tomato: Turn heat up to medium high. Add tomato paste and tomato pulp, cook for 2 minutes.

Coconut milk: Add water, coconut milk, sugar, salt and chilli powder if using. Stir, then bring to simmer, low heat so it's bubbling gently.

Simmer 2 minutes, add tomato & green chilli: Simmer for 2 minutes, add tomato and green chilli.

Simmer 3 minutes: Simmer for another 3 minutes, stirring every now and then - sauce should be thickened (not watery), it gets looser again when fish is added (because fish will drop some juices = even more flavour!).

Add fish: Add fish, stir, cook for 3 to 4 minutes until fish easily flakes.

Garnish & serve: Remove from stove and transfer to serving bowl. Garnish with coriander/cilantro and fresh green chillies if desired. Serve with basmati rice!