Heat the olive oil in a large frypan over medium high heat. Add pumpkin, carrots, onion and garlic then cook stirring until lightly browned. Put it all in the slow cooker. Add the stock and sage and salt. Cover and cook on Low for 4 hours.
When cooked, puree the soup with a stick blender until its at the consistency you like. Stir in the butter until it melts, adjust the seasoning and serve.