Clean, peel, wash and cut the celery into large pieces. Peel and chop the onion.
Heat the butter in a pot, fry the onion and celery in it while stirring. Add stock, bring to the boil, cover and cook for about 30 minutes until soft.
Mash the celery in the stock with a potato masher or puree with a hand blender, stir in the crème fraîche. Season to taste with salt and pepper.