Heat oil in a large Dutch oven or soup pot over medium heat. Add onion, garlic, celery and carrot and sauté until fragrant and the onion is tender, about 4 minutes.
Add mushrooms and continue to stir for another 3-4 minutes just until the mushrooms start to brown.
Add barley and vegetable broth, season with salt and pepper and bring to the boil. Reduce heat and add bay leaf then cover and let simmer for 45 minutes until the barley is soft and has tripled in size. The barley should be soft but still chewy.
Remove bay leaf, transfer to serving bowls, garnish with fresh parsley and serve.