This beautiful flecked-green soup is a bowl of Spring flavours. And like many a Spring day it can be warm or chilled. For a heartier meal I like to serve the soup with slices of a strong, hard cheese on oatcakes.
1 onion, finely chopped
1 stick of celery, finely chopped
1tbsp plain flour
300ml whole milk
50ml pale cortado sherry
half a lemon
50ml double cream (optional)
a handful of shelled walnuts, toasted and lightly crushed
Melt the butter in a large pan. Put into that the chopped onion and celery and cook on a low heat until they soften but are not turning brown. While that is happening peel and dice the potatoes. Stir the flour into the softened onion and celery; then stir in the potatoes, milk, sherry and 300ml water. Season. Bring to the boil then put the lid on and simmer for 25 mins or until the potatoes are very tender.
Bring a separate large pan of water to the boil and blanch the watercress sprigs in it for just 15 seconds, making sure to keep a few sprigs back for garnish. Drain the wilted watercress and refresh in very cold water. Squeeze the excess water out of the leaves, then stir them into the pan of soup stock.
Liquidise the soup until as smooth as you prefer. Give it a squeeze of lemon and add the cream if it needs thickening. Check the seasoning.
Serve warm or cool; garnished with the toasted walnuts and the reserved watercress sprigs.
(This recipe originally featured for Borough Market.)