Peel each prawn: Pull of its head and then grab the legs to pull away its shell; reserve the shell. Remove the bowels (the black vein) on its back with a paring knife. Set aside 8 prawns, the prettiest, covered in the fridge, for garnish.
Heat the oil in a large saucepan and fry the prawn shells for about 5 minutes, stirring. Add the onion,