Cut the squash in half lengthwise and scoop out the seeds.
Place squash flat side down on cutting board and cut each half lengthwise.
Cut the quarters crosswise to make 2-inch cubes. Stir briefly, reduce heat to medium low and simmer for 18 minutes, until the squash is just cooked through. Puree the soup using a blender or food processor in four batches, using equal parts solids and liquid to help make a smooth puree. Pour each batch of pureed soup into a large serving bowl or medium saucepan if not serving right away. To serve ladle soup into bowls and garnish each bowl with a dollop of yogurt and a sprinkling of parsley.