Melt the butter in a pan and cook the shallots and garlic over a low heat until soft but not
coloured. Add the cucumber and cook gently, stirring frequently, until tender.
Stir in the milk, bring almost to boiling point, then lower the heat and simmer for 5 minutes.
Tip the soup into a blender or food processor and puree until very smooth. Season to taste.
Pour the soup into a large bowl and leave to cool. When cool, stir in the prawns, chopped herbs
and cream. Cover, transfer to the refrigerator and chill for at least 2 hours.
To serve, ladle the soup into four individual bowls, top each portion with a dollop of creme
fraiche, if using, and place a prawn over the edge of each dish. Scatter over a little extra
chopped dill and tuck two or three chives under the prawns on the edge of the bowls to garnish.
Serve at once.