Finely chop the onion and dice the potatoes.
Heat the oil in a saucepan, then gently sauté the onion for 5 minutes or until softened.
Add the potatoes, stir for a minute, then add the stock and bring to the boil.
Place the lid on and simmer for 15 minutes or until the potatoes are tender.
Stir the watercress, put the lid on and simmer for a couple of minutes until the watercress has just wilted.
Turn off the heat, then blend the soup in a food processor or with a stick blender until smooth and silky.
Season to taste and serve with cream