How to Make It
In a large saucepan, combine the stock, water, barley and thyme. Season with salt and pepper and bring to a boil. Cover and cook over low heat until the barley is nearly tender, about 18 minutes.
Meanwhile, in a large nonstick skillet, heat the oil. Add the mushrooms and shallot, season with salt and pepper and cook over high heat until tender and browned, about 8 minutes.
In a medium bowl, combine the sirloin, egg, bread crumbs, cheese, 1/2 teaspoon of salt and 1/4 teaspoon of pepper. Knead the mixture until blended, then roll it into sixteen 1-inch balls.
Add the meatballs and mushrooms to the soup and simmer over moderate heat until the meatballs are cooked through and the barley is tender, about 8 minutes. Discard the thyme. Stir the parsley into the soup and serve in bowls with sour cream.