In 3-quart saucepan, heat butter over medium-high heat. Add onion and carrots; cook and stir 3 to 4 minutes or until softened. Add flour; cook and stir until moistened. Gradually add half-and-half, cooking and stirring about 2 minutes or until smooth.
Stir in remaining ingredients. Cover; cook over medium heat about 15 minutes, stirring frequently, just until mixture comes to a boil.
If desired, top with additional chopped fresh basil leaves.