Photo: Ria Osborne
Place the porcini in a small bowl, cover with boiling water and leave to soak for about 5 minutes.
Melt the butter in a pan and add the garlic, the chopped flat mushrooms and seasoning. Cook, covered, for 5 minutes or until the juices are released
Add the chestnuts, porcini and their soaking liquor (minus any grit at the bottom of the bowl), plus the sherry and stock. Bring to the boil and simmer for 10 minutes. Blend; season to taste.
Meanwhile, prepare the exotic mushrooms and cook, in batches according to type, in the butter, until golden. Whip the cream to soft peaks.
Pour the soup into small cups and float some of the cream on top. Add a few exotic mushrooms and a drizzle of truffle oil, if using, to serve.