Put the oil, chillies, parsnips, onion, garlic and ginger into a large saucepan. Cut off and discard the top of each stem of lemon grass leaving the lower fatter 10cm or so. With the end of a rolling pin give these a good whack to flatten and pop them in too. Give the contents of the pan a quick stir put a lid on and place over a gentle heat. Leave them to sweat gently (maybe check once) for 10 minutes.
Next add the stock and soy sauce. Bring the mixture to the boil, then simmer gently for 10-15 minutes until the parsnips are very tender. Fish out and disgard the lemon grass, then stir in the coconut milk and liquidise the whole lot. Add more stock if it seems too think.
When you're nearly ready to eat reheat the soup, stir in the lime juice and serve. Sprinkle some of the chopped herbs on each bowl.
I absolutely love this soup, it's a magazine recipe I've had for years and make whenever I have some spare parsnips. The great thing is you can even use really huge ones that you have forgotten to harvest in time! I've made it for events at Abbey Gardens and it proved very popular.