Wash and deseed the chillies and cut into fine strips. Peel the lemon grass and ginger and chop finely.
Peel and wash the spring onions, pepper and carrot and cut into thin strips.
Pour 1l boiling water over the noodles in a bowl.
Fry the chilli, carrot, ginger and lemon grass in a pan in hot oil.
Add the vinegar and 900ml stock. Bring to the boil.
Add the spring onion and pepper strips.
Mix the remaining stock with the cornflour and stir into the soup.
Simmer for 5 minutes, stirring constantly.
Drain the noodles in a sieve and add to the soup.
Season with soy sauce and pepper.
Serve in small bowls topped with coriander.