1.Place celeriac, onion, celery, garlic, water and stock in a large saucepan. Bring to the boil. Reduce heat and simmer with lid on for 1 hour or until tender. Remove pan from heat and stand uncovered, 10 minutes.
2.Blend or process soup, in batches, until smooth. Return soup to pan and stir in cream, over medium-high heat, until hot. Season. Stir in juice.
3.Serve soup sprinkled with chervil and drizzle with oil.