Heat oil in a large Dutch oven over medium-high heat. Add chopped onion, celery, minced ginger, dried thyme, and saffron threads, and sauté 5 minutes. Add sweet potato and orange rind, and sauté 3 minutes. Add broth and red pepper, and bring to a boil. Cover, reduce heat, and simmer 25 minutes or until potato is tender. Place half of potato mixture in a blender or food processor, and process until smooth. Pour puréed mixture into a bowl. Repeat procedure with remaining potato mixture. Return puréed mixture to pan. Stir in the spinach, and cook until thoroughly heated.