Melt the butter in a saucepan and when foaming, add the onion and lemon grass and fry over a low heat until softened.
Peel and trim the beetroot. Cut into large chunks. Put into the pan with the stock. Season with salt and pepper and bring to boil, then lower the heat and simmer for 20 mins.
Puree the soup in a food processor or blender with the coconut milk.
Pour the soup into the rinsed-out pan, add the lime juice and season to taste with salt and pepper.
Reheat and serve in warmed soup bowls, each topped with a spoonful of creme fraiche and a little zest.