Place the prawn heads and shells, chilli flakes, salt and freshly ground black pepper, soy sauce and sesame oil into a saucepan and pour over the boiling water. Boil for 10-15 minutes, skimming off any impurities from the surface. Remove the heads and shells from the pan and discard. Add the lime juice to the soup, to taste.
To serve, pour the soup into a serving bowl and garnish with the orange segments, chopped coriander and sesame seeds.
