1 Heat the oil in a pan and sweat the shallots and potatoes for a few minutes. Deglaze with chicken stock and simmer until soft, about 30 minutes.
2 Add the cream and sour cream and simmer gently for a further 5-10 minutes. Puree the soup in a liquidiser and pass through a fine sieve. Season to taste with salt and pepper and strain the oyster liquor into the soup through a fine sieve.
3 Heat the butter and cook the oysters gently for 1 minute. Season with salt and pepper.
4 Ladle the soup onto warmed plates and add the oysters. Drizzle with truffle oil, shave a little truffle over each dish and serve garnished with chervil.