Heat olive oil in a Dutch oven over medium heat. Add leek and celery, and cook 7 minutes or until leek is wilted, stirring occasionally. Add potato and next 5 ingredients (potato through broth), and bring to a boil. Cover, reduce heat, and simmer 15 minutes or until vegetables are tender. Stir in arugula and spinach, and cook for 2 minutes or until wilted. Cool slightly. Place half of mixture in a food processor or blender, and process until smooth. Pour purée into a large bowl. Repeat procedure with remaining mixture. Return puréed mixture to pan. Stir in milk and sherry, and cook for 5 minutes or until thoroughly heated.