Heat oil in heavy large pot over medium-high heat. Add onions and celery; sauté until onions are golden, about 7 minutes. Add garlic; stir 1 minute. Add broth and next 5 ingredients; bring to boil. Reduce heat; cover and simmer until squash is tender, about 15 minutes. Transfer 3 cups soup to blender; cool slightly, then puree until smooth. Return puree to pot with soup. Simmer until heated through, about 5 minutes. Season with salt and pepper.
Ladle soup into 6 bowls. Sprinkle each with 1 tablespoon basil and serve.
