Heat the sesame oil in a large saucepan over a moderate heat until hot. Add the chilli, then the spring onion immediately after. Stir for 1 minute, then add the fish stock. Bring to a simmer over a reduced heat and add the peppercorns.
Add the prawns and allow them to simmer for 6-8 minutes until cooked. Once cooked, adjust the seasoning with the fish sauce and soy sauce. Stir in the watercress, allowing it to wilt before spooning into serving bowls.