Heat the oil in a large saucepan, then add the onion and cook gently for 5–10 minutes, until soft. Add the potato and cook for a further 5 minutes.
Stir through the watercress, reserving a few sprigs to garnish.
Pour over the stock and simmer for 20 minutes. Using a hand-held blender, whizz until smooth.
Season with salt and freshly ground black pepper and serve in warmed bowls.
Top each with a spoonful of crème fraîche and the reserved watercress.