Heat the olive oil in a large pot, add the onion, carrots, and celery, and cook for about 5 minutes.
Add the sliced mushrooms and continue cooking for 6 to 7 minutes until their moisture begins to evaporate.
Stir in the garlic and cook for 30 seconds, then add the rinsed barley.
Pour in the vegetable broth and season with thyme, bay leaf, salt, and black pepper.
Bring the soup to a boil, reduce the heat, cover, and simmer for 35 to 40 minutes until the barley is tender.
Remove the bay leaf and add a little extra broth if the barley has absorbed too much liquid.
Stir in the fresh parsley, taste the seasoning, and serve the soup while warm.