Melt the butter in a soup pot over medium heat. Add the chopped onion and sauté for 5 minutes.
Add the butternut squash and chicken broth. Bring to a boil, then lower the heat and simmer for 30 minutes, or until the squash is soft.
Blend with an immersion blender (or food processor or blender) until completely smooth. Stir in salt and pepper to taste.
Serve in a bowl topped with fresh parsley.