Heat the olive oil in a saucepan and cook the onion and potato over a medium-low heat for 20 minutes. Add the watercress (keep a little back to use for garnish) and water and cook for 5 minutes until the watercress wilts.
Add the stock, seasoning and nutmeg, bring to the boil and gently simmer for 15 minutes. Blend with a hand-held blender until smooth. Check the seasoning, scatter with the reserved watercress and serve at once.