1.Make sweet cucumber and peanut sauce. Cook sugar and the water in small saucepan, stirring, without boiling, until sugar dissolves. Bring to the boil, simmer, uncovered, until syrup is reduced by half. Remove syrup from heat, stir in juice, fish sauce, chilli and ginger, cool. Just before serving, stir in remaining ingredients
2.Meanwhile, spray garfish both sides with cooking-oil, cook, in batches, on heated oiled grill plate until browned and cooked through.
3.Serve garfish with sauce and, if desired, thick char-grilled lime slices.
the sweet cucumber and peanut sauce can be made up to four days ahead.