• Wash and trim the leeks, then cut into thin rings. Finely chop the onion. Peel and dice the potato.
• In a saucepan cook the onion and leeks in the butter or margarine until softened. Add the potato and stock. Bring to the boil, reduce the heat and simmer gently for 20 minutes.
• Add the milk. Blend the soup in a liquidiser until smooth. Transfer to a bowl and whisk in the cream, yogurt, salt and pepper and Tabasco.
• Leave the vichysoisse to cool. When cold, chill for at least 1 hour.
• Serve the soup in individual bowls and garnish each serving with freshly chopped chives.

Cook's Tip

• To clean leeks thoroughly, trim roots and tough green leaves off leeks. Slit leeks lengthwise to centre, then hold root end uppermost under cold running water to wash out dirt.