Carefully wash the courgettes and flowers. Very gently remove all the flowers from the courgettes.
Fillet the John Dory. Skin each fillet and break them into their 3 natural pieces each side. Cut each of the long pieces, 4 to each fish, in half at an acute angle, and put to one side. The 4 small pieces are to be used for the mousse.
Cut the small fillets into small pieces and place in a blender or food processor along with the salt and process until a smooth paste. Add the egg white and mix in thoroughly until it stiffens. Remove from the blender and rub through a sieve. Place in a bowl set on crushed ice and gradually add two-thirds of the cream, followed by the sherry and seasoning. Test the mousse. If the mousse is a little firm, add more cream; the mousse needs to be really soft for this recipe.
Place the mousse in a piping bag with a fairly fine nozzle. Carefully fill each flower about half full with the mixture. Fold the ends of the petals over to form a seal. Place each flower, folded end down, in a steaming basket along with the courgettes and steam for 4 minutes.
While these are cooking, heat the oil in a frying pan and, when the oil is hot, add