In a large saucepan, saute onions and celery in oil until tender. Add 4 cups broth and potatoes; bring to a boil. Reduce heat; cover and simmer for 15-20 minutes or until potatoes are tender., Cool slightly. In a blender, process soup in batches until smooth. Return to pan; add the remaining broth and bring to a boil. Add broccoli, salt and pepper. Reduce heat; simmer, uncovered, 8-10 minutes or until broccoli is tender.