Heat oil and butter in a large pot over medium heat. Add onion, celery and carrot; saute until soft, but do not brown. Stir in flour and cook another minute. Add remaining ingredients, bring to a boil, then reduce heat to medium-low and let simmer, covered, for 15-20 minutes. Stir in cream. Transfer to a blender and puree (you will probably have to do this in two batches). Garnish with julienned basil leaves and serve.