400g/14oz Jerusalem artichokes, peeled and cut into small pieces
1 tbsp chopped fresh chives
70²µ/2½´Ç³ú onions, sliced
250ml/9fl oz chicken stock
salt and freshly ground black pepper
few drops truffle oil
30g/1oz unsalted butter
50²µ/1¾´Ç³ú butter
1 bouquet garni