- Heat the oil in a saucepan, add the onion and potato and fry for 2-3 minutes. Add the stock to the pan, bring to the boil and simmer for 15- 20 minutes until the potato is tender.
- Remove the pan from the heat, add the watercress and blend with a stick blender or food processor until smooth.
- When fully blended add the yogurt, season well and heat through gently, without boiling, then blend for 1 more minute.
- Serve topped with a swirl of yoghurt and a sprinkle of black pepper.
- A ladleful of this soup makes a great sauce served with grilled fish or vegetarian dishes such as tofu kebabs and rice.
- Freezing instructions: Can be frozen in small pots for individual portions. Defrost in a microwave and slowly until piping hot.