Mushroom Soup

Name: Mushroom Soup
Cuisine: French
Ingredients: 6 tbsp/75 g butter; 1 small onion, thinly sliced; 12 ounces/340 g button mushrooms; 4 cups/900 ml light chicken stock or broth; 1 sprig of flat parsley; Salt and pepper; N/A 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); Medium saucepan; Wooden spoon; Blender