6 tablespoons/75 g butter
1 small onion, thinly sliced
12 ounces/340 g white mushrooms
4 cups/900 ml vegetable or chicken broth
1 sprig of flat parsley
1/2 teaspoon salt
1/2 teaspoon freshly ground black pepper
2 ounces/56 ml high-quality sherry (don't use the cheap grocery-store variety; it's salty and unappetizing and will ruin your soup - Bourdain's advice)