Add the onions to the saucepan and cook over low heat, stirring occasionally, until transparent, about 6 minutes. Add the Ham Stock and bring to a boil over high heat. Stir in the squash flesh and bring back to a boil. Transfer the soup to a blender or food processor and puree until smooth. Strain into the saucepan and season with salt and white pepper. If necessary, stir in a little hot water to thin the soup; keep warm.