Heat the oil in a large soup pot over medium heat. Add the onion, carrots, celery and garlic; cook and stir until onions are tender and transparent. Stir in mushrooms and continue to cook for a few minutes. Pour in the chicken broth, thyme a bit parsley and barley. Bring to a boil, then reduce heat to low. Cover and simmer until barley is tender, about 50 minutes. Season with salt and pepper before serving. Garnish with chopped parsley.