Leek and potato soup (Vichyssoise)

Heat the butter in a large saucepan and add the leeks and onions. Cook until tender but not coloured.

Add the potatoes and stock, and season.

Bring to the boil, reduce the heat and simmer for 15-20 minutes until the potatoes are tender.

Puree in batches in a blender. Return to the pan and bring to the boil.

Whisk the cream into the soup. Leave to cool, then taste and adjust seasoning.

Refrigerate until chilled.

Serve sprinkled with chives.