Beef broth or stock, 4cups (32fl oz/1l), homemade or purchased
Boneless beef sirloin, ½lb (250g), cut into thin slices and then finely chopped
Cook the barley
In a saucepan over medium-high heat, bring 3cups (24fl oz/750ml) water to a boil. Add 1teaspoon salt and the barley and return to a boil. Reduce the heat to low, cover, and cook just until tender, about 1½ hours. Drain and set aside.
Prepare the soup base
Meanwhile, in a large saucepan over medium heat, melt the butter. Add the onion and mushrooms and cook, stirring frequently, until the onion is translucent and the mushrooms have released their juices, about 5 minutes. Add the broth and beef, reduce the heat to low, and simmer, uncovered, for about 10 minutes.
Finish the soup
When the barley is ready, add it to the soup base and simmer over medium heat for 5 minutes to blend the flavors. Season to taste with salt and pepper. Ladle into bowls and serve.