Bruise the stick of lemon grass with the back of a knife. Chop half of the ginger roughly. Pick the leaves from the coriander and keep to one side and use the stalks for the stock. Add these ingredients and the lime leaf to the stock and bring to the boil. Once boiling reduce to a simmer.
Finely slice the onion. Slice the shiitake mushrooms. Finely chop the remaining ginger, the garlic clove and the chilli.
Remove the heads and shells from the prawns and then carefully cut down the spine to remove the digestive tract.
Heat a saucepan and add a splash of peanut oil. Cook the onions until soft and then add the mushrooms. Cook for a further minute before adding the prawns. Once the prawns begin to turn pink add the garlic and ginger. Cook for a further minute and pour in the stock, through a sieve and discard the lemongrass and ginger.
Add the noodles and simmer until the prawns are cooked through and the noodles are soft.
Finish the soup with a squeeze of lime, the chilli and add fish sauce to taste. Just before serving add the coriander leaves and the bean sprouts.
«For a vegetarian alternative, replace the prawns with tofu and use a vegetable stock.»