Peel, wash and chop sweet potatoes, celery and carrots. Peel and roughly chop the onion. Heat butter in a pot. Fry potatoes, celery, carrots and onion.
Add stock, bring to the boil and cook for about 20 minutes. Wash the coriander and chop finely, except for a little to garnish. Cut the sausage into thin slices. Heat oil in a pan. Fry the sausage for about 2 minutes, turning it over.
Coarsely mash the vegetables, season with salt and pepper. Stir in coriander. Arrange in small bowls, add slices of sausage and garnish with coriander.