Melt butter in a large saucepan over medium heat; add onion, and cook 3 minutes or until tender, stirring occasionally. Add carrot and broth. Bring to a boil; reduce heat, and simmer, uncovered, 45 minutes or until very tender.
Place half of carrot mixture in a food processor or blender; process until smooth. Pour pureed mixture into a large bowl; repeat procedure with remaining carrot mixture.
Return pureed mixture to saucepan; add water and remaining 3 ingredients. Bring to a simmer over medium heat, and cook 2 to 3 minutes or until mixture is thoroughly heated.