From Bourdain's Les Halles book. You must try this recipe. It is incredibly simple to make yet it is extremely flavorful.
2 tablespoons unsalted butter
1 yellow onion, thinly sliced
4 tablespoons unsalted butter
12-14 ounces button mushrooms (I used 10 oz baby portabella and 4 oz shiitake 'cause that's what I happen to have at hand), whole (stems removed if using shiitake)
4 cups light chicken stock (I used Kitchen Basics Chicken Stock; light sodium chicken broth will probably work but don't tell Bourdain or he'll chase you down!)
1 sprig parsley
salt and pepper
2 ounces of Sherry (the real deal not the crappy salted junk from the market) (I used tawny port because that's what I had, seemed to work well enough for me)
In a medium saucepan, melt 2 tablespoons of butter over medium heat and add the onion. Cook until translucent (a few minutes).
Add the 4 tablespoons of butter and mushrooms and cook for 8 minutes or so, until the mushrooms are sweating. Make sure that the onions do not brown.
Stir in chicken stock and parsley. Bring to a boil and then lower the heat. Simmer for 1 hour or so. The recipe didn't state whether to cover or not, I left it slightly covered. Just make sure it is simmering slowly.
After an hour, remove parsley and blend on high speed IN BATCHES. Make sure that you hold down the cover of the blender VERY FIRMLY or you will be splashed and burned by the mushroom soup. Believe me, it almost happened to me.
Return blended soup to the pot, season w/ salt and peper and bring to a simmer again. Add the sherry and mix well.
Serve immediately or reheat for the next day.