50g/2oz/4 tbsp butter
2 onions, roughly chopped
900g/2lb leeks, chopped
1.2 litres/2 pints/5 cups vegetable stock
150ml/¼ pint/scant ⅔ cup double (heavy) cream
225g/8oz potatoes, cubed
salt and ground black pepper
1 Melt the butter in a large pan over a low heat. Cook the onions and leeks, stirring occasionally, for 10 minutes or until softened.
2 Stir in the stock, cream and potatoes and bring to the boil. Reduce the heat, cover the pan and simmer gently for 30–40 minutes until the vegetables are tender.
3 Blitz the soup in batches in a blender or food processor until smooth. Season to taste with salt and pepper.
4 Transfer the soup to a large bowl, cover and chill in the refrigerator for 6 hours or overnight.
5 Ladle the soup into chilled bowls and serve.