Heat the oil in a large saucepan and gently fry the onion and fennel for about 10 minutes until they are very soft and beginning to brown.
Add the chorizo, potatoes and stock and bring to the boil. Reduce the heat, cover with a lid and cook gently for 20 minutes until the potatoes are very tender.
Blend the soup until fairly smooth in a blender or food processor. Stir in the coriander and crème fraîche and heat through gently for a couple of minutes. Season with salt and pepper to taste and serve in small, warmed cups.