Dice the onion and sauté in the butter over medium heat until translucent. Pour in the tomatoes, coconut milk and vegetable stock. Add the freshly chopped coriander and season the soup with salt, pepper and sugar. Bring to the boil briefly, then simmer for about 10 minutes. If necessary, puree the tomatoes and, depending on the desired consistency, add vegetable stock. Bring to the boil again and season to taste.
Serve with a spoon of creme fraiche and fresh coriander leaves.