Meanwhile, melt remaining butter in large saucepan. Add onions and celery; cook 5 min. or until softened, stirring frequently. Stir in sherry; cook and stir 30 sec. Add potatoes and chicken broth; stir. Bring to boil; simmer on medium-low heat 10 min. or until potatoes are tender, stirring occasionally. Remove 2 cups soup from saucepan. Blend in blender or food processor or until smooth. Return to saucepan.