Soak the pearl barley in water for several hours or overnight.
In a large pot, heat a bit of olive oil and sauté the onion until it becomes translucent.
Add the carrots, parsnips, and celery. Cook for a few minutes.
Add the drained barley, vegetable broth, bay leaves, and thyme. Bring to a boil, then reduce the heat and let simmer for about 30 minutes, or until the barley is tender.