Pour the fish stock into a saucepan, add the lemon grass, lime leaves or peel and chillies. Simmer for 10 minutes. Strain the liquid into a clean saucepan. Reserve a little of the red chilli from the sieve and discard the remaining seasonings.
Add the lime juice and fish sauce to taste to the stock with the bamboo shoots, mushrooms and reserved chilli. Simmer for 5 minutes. Ladle the soup into individual bowls and sprinkle with the spring onions. Garnish with fresh red chilli slices.