Slash the fish through the flesh on both sides with diagonal cuts - five on each side - then rub all over with salt and put on one side. Pound the garlic, ginger and birdseye chillies with about 1 tsp of salt to make a paste. Crush the mild chillies into the paste, leaving them with a little texture. Stir in the sugar, fish sauce and about 2 tbsp water and spoon into a small pan. Bring the sauce to a boil, simmer gently until syrupy, then stir in the lime juice and leave to one side. Heat a deep pool of oil in a wok or pan, then test it with a cube of bread - it should fry and brown gently. Slip the fish into the hot oil (the oil may splash a little) and fry, turning once, until crisp and cooked - 5-8 minutes, depending on the size of your fish. Drain the fish and slip it on to a serving plate, then reheat the sauce and pour it over - and stick a thatch of the greenery and red chilli on top. Eat with rice.