Beef & Barley Soup

Name: Beef & Barley Soup
Yield: Makes 4 Servings
Ingredients: Salt and freshly ground pepper; Beef broth or stock, 4 cups (32 fl oz/1 l), homemade or purchased; Unsalted butter, 2 tablespoons; Pearl barley, ¼ cup (2 oz/60 g); Boneless beef sirloin, ½ lb (250 g), cut into thin slices and then finely chopped; Button mushrooms, ½ lb (250 g), finely chopped; Yellow onion, 1 small, chopped