Pour boiling water over shiitake mushrooms; let stand 10 minutes. Drain shiitake mushrooms, reserving liquid. Remove and discard stems, and slice shiitake mushrooms. Set shiitake mushrooms aside.
. Heat oil in a Dutch oven over medium heat 1 minute. Add onion; cook 9 minutes, stirring often. Add button mushrooms, salt, and pepper. Cook 5 minutes, stirring often. Add barley, garlic, and shiitake mushrooms. Cook 2 minutes, stirring constantly. Add chicken broth and reserved mushroom soaking liquid, discarding sediment, if necessary. Cover and bring to a boil. Reduce heat to medium-low; simmer 30 minutes or until barley is tender. Sprinkle with chives.