1.Heat oil in a large saucepan over medium heat. Stir fennel over heat until soft. Add leek, celery and swede. Cook, stirring occasionally, about 15 minutes or until vegetables begin to caramelise. Stir in garlic and continue stirring until fragrant.
2.Coarsely grate potato, add to pan with stock and water; bring to the boil. Reduce heat and place lid on pan. Simmer about 20 minutes or until vegetables are tender. Cool 10 minutes.
3.Meanwhile, to make caramelised fennel, melt butter in a medium frying pan. Stir fennel over heat for about 10 minutes or until tender. Stir in sugar and continue stirring for 5 minutes or until caramelised. Add sherry and cook 1 minute or until mixture thickens. Keep warm.
4.In batches, blend or process soup until smooth. Season to taste. Strain soup into pan, add half the cream. Heat soup over medium heat, stirring occasionally, until hot.
5.Trim scallops; season. Melt butter in a large frying pan. In batches, sear scallops, basting with butter until edges begin to turn golden. Add juice to pan, spoon mixture over scallops.
6.Ladle soup into serving bowls; top with caramelised fennel and scallops. Drizzle with remaining cream, and sprinkle with reserved fennel fronds.