Finely chop the lemon grass stalk and mix with the garlic, coconut and green chillies. Smear this dry mixture over each plaice, then cover and leave to marinate in the refrigerator for 2 hours or overnight.
Make the coconut and tamarind sambal by gently frying the onion and garlic in the oil in a large frying pan until softened. Add the coconut with the red chilli, stir to coat in the oil and cook for 2–3 minutes. Pour the measurement water over the tamarind pulp in a heatproof bowl and stand for 10 minutes to dissolve.
Strain the juice from the tamarind pulp, mashing as much of the pulp through the sieve as possible. Add this juice to the pan with the sugar and simmer gently for 5 minutes. Add the vinegar, remove from the heat and leave to cool. When cold, stir in the chopped coriander. Turn into a bowl and wipe the pan clean.
Heat the oil in the pan and gently fry the plaice 2 at a time in the hot oil, turning once. After 6–8 minutes, when they are golden brown and cooked, remove from the oil and drain on kitchen paper. Keep warm while cooking the remaining fish. Serve the fish piping hot with the coconut and tamarind sambal.