Trim beetroot stems and leaves. Wearing gloves, peel and chop beetroot. Heat oil in a saucepan over medium-high heat. Add onion, carrot and celery. Cook, stirring, for 5 minutes or until onion is tender. Add beetroot, potatoes, bouquet garni, stock and 1 cup cold water. Bring to the boil. Reduce heat to medium-low. Simmer, partially covered, for 1 hour or until beetroot is tender. Cool slightly. Remove and discard bouquet garni. Process soup, in batches, until smooth. Return soup to pan over low heat. Cook, stirring, for 4 to 5 minutes or until heated through. Season with salt and pepper. Ladle soup into bowls. Top with sour cream. Sprinkle with chives. Serve.