For the curry paste, place all Ingredients in a mortar and grind to a smooth paste.
Skin the fish and cut into 1 inch (2.5cm) pieces. Marinate the fish pieces with the salt, lemon juice and soy sauce for about 30 minutes.
Trim the scallions (spring onions) and slice into rings. Trim the zucchini (courgette), cut in half length ways, then thinly slice. Finely chop the kaffir lime leaves.
Heat the coconut milk over a low heat and simmer for 5 minutes. Heat the oil in a wok and fry the fish for about 4 minutes. Pour in the coconut milk and the curry paste and the rest of the marinade (if you don't use all the curry paste, put the remainder in a jar and keep it in the refrigerator). Reduce the heat and simmer the fish for 3 minutes.
Add the scallions, zucchini, some basil leaves and lime leaves and cook for a further 3 minutes.
Scoop the curry onto warmed plates and garnish with the remaining basil leaves. Serve with basmati rice.