For the seasoning paste: Tip all the ingredients into a food processor and blend to a smooth paste. This paste tastes best if left for a few days to mature before using.
For the souscaille: combine one rounded tablespoon of the seasoning paste with the lime juice, sugar, celery leaves, coriander, red pepper and olive oil. Taste for seasoning – adding more sugar or lime juice if needed. Aim for a tart, tangy flavour.
For the fish or prawns: combine the lime rind and juice with the garlic, red pepper, spring onion and coriander and spoon over the fish or prawns and leave on one side for 5 minutes. Remove from its marinade and sprinkle with curry powder just before frying.
Heat the oil in a sturdy frying pan set over a medium heat and fry the fish for 3 minutes, skin side facing downwards. Flip the fillets over and cook for a further minute until it flakes easily. If you are using prawns, fry for 3-4 minutes, stirring all the time, over a moderate heat, until they have turned pink. Remove and set aside.
Tip the tomatoes into the same pan while still on the heat. Add the butter and sharpen with a squeeze of lime.
Spoon a dollop of souscaille sauce over the fish and serve any extra on the side. Accompany with the warmed tomatoes.