Heat the oil in a saucepan, add the peppercorns, lemon grass, cinnamon, star anise and ginger and cook for 1 minute to release their flavours. Cut the stems from the coriander and add the stems to the pan with the stock. Bring to the boil, stirring then cover and simmer for 40 minutes.
Strain the stock and return to the pan. Stir in the fish sauce and lime juice. Cook the noodles in a pan of boiling water as directed on the pack then drain and divide between 6 small bowls. Add the steak to the soup and cook for 1–2 minutes. Divide the bean sprouts, spring onions and chilli between the bowls then ladle the soup on top and finish with the remaining coriander leaves, torn into pieces.