Melt butter in a large saucepan over medium-high heat and cook until lightly browned. Add the mushrooms and cook, stirring , until the liquid evaporates--about 10 minutes. Sprinkle in the flour, then stir in and keep stirring for about a minute to let the flour develop. Whisk in the the stock, sherry, and mustard--then grate in the potato into the broth. Bring to a simmer and cook for 10 minutes. Season with salt and pepper. Ladle into bowls, drizzle a little sherry on top and garnish with diagonally sliced green onions.