Heat a large saucepan over medium heat. Spray with oil. Cook the onion, stirring occasionally, for 3 minutes or until soft. Add carrot and cook, stirring often, for 5 minutes or until just soft.
Add stock. Bring to boil. Peel 2 strips of rind from the orange. Add to pan. Reduce heat to medium-low. Simmer for 20 minutes or until carrot is tender. Remove and discard orange rind.
Set aside for 5 minutes to cool. Use a stick blender to puree until smooth. Juice the orange. Add to soup. Place over medium heat and cook for 2 minutes or until heated through. Top with parsley and bread croutons (optional).