In a large saucepan, heat oil over medium-high heat. Add onion and celery; cook and stir until tender, 2-4 minutes. Add remaining ingredients, except for optional fresh basil. Bring to a boil. Reduce heat; simmer, uncovered, 10 minutes to allow flavors to blend., Puree soup using an immersion blender. Or cool soup slightly and puree in batches in a blender; return to pan and heat through. If desired, top with fresh minced basil.