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