Cook and stir onion in butter in 4-quart Dutch oven until tender. Stir in broth, squash, 2 sliced pears, thyme, salt, white pepper, and coriander. Heat to boiling; reduce heat. Cover and simmer until squash is tender, 10 to 15 minutes. Pour about half of the soup into food processor workbowl fitted with steel blade or into blender container; cover and process until smooth. Repeat with remaining soup. Return to Dutch oven; stir in whipping cream. Heat, stirring frequently, until hot. Serve with sliced pear and pecans.