In a large pot, heat the oil. Add the onion, leeks and garlic and cook over moderate heat, stirring occasionally, until tender, about 5 minutes. Stir in the barley. Add the vegetable broth, water, thyme and bay leaves and bring to a boil. Add the celery root and parsnips and season with salt and pepper. Simmer over moderately low heat until the barley and root vegetables are tender, about 40 minutes.