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. Add the stock, sherry, and mustard--then grate in the potato. 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.