Melt the butter in a heavy-based pan and cook the leeks and shallots gently, covered, for 15-20
minutes, until soft but not browned.
Add the potatoes and cook, uncovered, for a few minutes. Stir in the stock or water, 5ml/l tsp
salt and pepper to taste. Bring to the boil, then reduce the heat and partly cover the pan. Simmer
for 15 minutes, or until the potatoes are soft.
Cool, then process the soup until smooth in a blender or food processor. Strain the soup into a
bowl and stir in the cream. Taste and adjust the seasoning and add a little iced water if the
consistency of the soup seems too thick.
Chill the soup for at least 4 hours or until very cold. Taste the chilled soup for seasoning and
add a squeeze of lemon juice, if required. Pour the soup into bowls and sprinkle with chopped
chives. Serve immediately.