Bring chicken stock to a boil in a large saucepan. Melt butter in a large frying pan over medium heat. Add onion, and fry for 5 minutes. Add the barley, thyme, bay leaf and 2 cups of the hot stock. Bring to a boil, reduce heat to low, and simmer until most of the stock is absorbed, about 10 minutes.
Pour in remaining stock 1/2 cup at a time, stirring and allowing it to be absorbed before adding more. This process takes about 50 minutes.
Meanwhile, heat olive oil in a large frying pan. Fry mushrooms over high heat, cook and stir until softened. Add garlic, and cook for about 3 more minutes. Stir into the barley mixture with parsley. Remove bay leaf, and serve.