Heat the coconut oil in a large saucepan or wok over medium heat and add the onions, chilli, ginger, garlic, curry leaf and mustard seeds, fry for 12-15 mins until the onion is soft.
Stir in the tomato purée, chilli and turmeric and cook for another 1-2 mins. Pour in the coconut milk and 200ml water, simmer the sauce for 20 mins, then blitz with a stick blender until smooth.
Pour the sauce back into the saucepan and bring up to a simmer. Stir in the cauliflower and potato, cover and simmer for 15 mins.
Add the fish and poach for about 5 mins, or until cooked through.
To serve, scatter the spring onions, coriander and peanuts over the curry and squeeze over a little lime juice.