Heat the oil in a large saucepan, then put the onion, chillies and ginger in to cook for 2 minutes. Add the mushrooms and cook for 3 minutes, stirring occasionally.
Pour in the stock and rice wine or white wine vinegar. Add the chopped lemongrass, bring the broth to the boil and then simmer for 10 to 12 minutes.
Add the prawns, noodles and coriander then bring the soup back to the boil. Reduce the heat and simmer for 3 to 4 minutes.
Divide the ingredients and broth evenly between 4 bowls, then serve the prawn and noodle soup garnished with the finely sliced spring onions.