Cook the barley according to package instructions and set aside.
In a large pot, sauté the onion, garlic, carrots, and celery in olive oil until soft.
Add the mushrooms and cook until they release their juices.
Pour in the vegetable broth and add the thyme.
Bring to a boil, then reduce to a simmer for 20 minutes.
Stir in the cooked barley and season with salt and pepper.
Serve warm, garnished with fresh parsley.