Pour the stock into a saucepan then add the orange rind and juice, sherry, spice powder and ginger. Stir in the soy sauce and plum sauce then bring to the boil, stirring. Cover and simmer gently for 15 minutes.
Add the duck, spring onions and pak choi and simmer for 5 minutes. Taste and add a little salt and pepper if needed then ladle into bowls.