In a Dutch oven, cook chicken in 1 tablespoon oil until no longer pink. Remove and set aside.
In the same pan, saute the leeks, celery and carrot in remaining oil for 4 minutes. Add mushrooms and garlic; cook 2 minutes longer. Stir in the broth, water, seasonings and chicken. Bring to a boil. Reduce heat; cover and simmer for 15 minutes.
Stir in barley and return to a boil. Reduce heat; cover and simmer for 10-15 minutes or until barley and vegetables are tender. Discard bay leaf.