Here is the recipe that I have adapted from Anthony Bourdain's Les Halles Cookbook.
1 small onion, thinly sliced
12 ounces of Portabella mushrooms (I also add few little dried Chanterelle mushrooms)
4 cups light chicken stock or broth
1 sprig of flat parsley
Salt and pepper
2 ounces high-quality sherry
Melt 2 T of butter in a stock pan. Saute onions till translucent, then add mushrooms and rest of butter, let them sweat for about 10 minutes (do not let the onions brown) then add chicken stock(and dried Chanterelles) and parsley bring to a boil then immediately reduce heat and let simmer for an hour. After an hour, remove from heat, remove parsley and let cool, then place in blender, add sherry and blend till smooth. After placing the soup in individual bowls I drizzle a little truffle oil over the surface before serving, as Anthony says to do. ;0)
I promise you this is delicious!