This is an adaptation/simplification of a Basque recipe published in Saveur magazine, issue number 65. It is so easy and so delicious that I had to share it.
You should use whatever kind of fresh mushrooms look the best in the market. Wild ones are great but not necessary. I think it's best with brown mushrooms like cremini or portobello, or porcini or morels if you can get them fresh.
Don't wash the mushrooms before sauteeing them; clean any dirt off with a soft brush or a paper towel. You can get a mushroom brush made for the purpose at any kitchen supply store, and probably in the supermarket as well.
1/4 lb butter
2 scallions (white parts only)
1/2 lb fresh mushrooms
1 qt beef stock
1 cup heavy cream
Fresh-ground black pepper
Clean and slice the mushrooms. Chop the scallions (white parts only). Melt 2 tablespoons of the butter in a heavy pot (4-qt. or larger) over medium heat. (There are 8 tablespoons of butter in 1/4 lb.). Saute the scallions and the mushrooms in the butter, with a little salt, stirring frequently with a wooden spoon, until the mushrooms turn soft and begin to brown, 5-10 minutes. Reserve about a quarter to a third of the mushrooms in a side dish for later use.
Add beef stock to the remaining mushrooms, bring to a boil, then simmer for 20 minutes. Pour stock and mushrooms into a blender and puree until fairly smooth.
Return puree to cooking pot and add cream. Heat over medium heat until hot but not boiling. Add remaining butter 1 tablespoon at a time, whisking in each lump of butter until it has melted before adding the next one. Correct seasoning by adding salt to taste (if your beef stock was salted to start with, you may not need any more).
Distribute reserved sauteed mushrooms among 4 soup bowls. Pour hot soup over mushrooms to fill each bowl. Serve at once. Grind fresh black pepper over soup at table.