Bring a large saucepan of water to the boil and pour in the pearl barley. Cook at a gentle simmer for 1 hour. Drain well.
Meanwhile, heat the oil in a large, heavy-based saucepan over a medium-low heat, add the onion, leeks and celery and fry gently for 8–10 minutes or until softened but not coloured. Add the root vegetables and cook for a further 5 minutes, stirring regularly.
Pour in the stock, add the bouquet garni and bring to the boil. Stir in the pearl barley, then reduce the heat and simmer for 25–30 minutes or until the vegetables and pearl barley are tender. Remove the bouquet garni and season to taste with salt and pepper. Ladle into bowls and serve with herby bread, if liked.