1. Combine first 6 ingredients in a 5-quart oval electric slow cooker.
2. Rinse turkey with cold water; pat dry. Sprinkle turkey with five-spice powder and salt. Heat a large nonstick skillet over medium-high heat. Add oil to pan; swirl to coat. Add turkey; cook 3 to 4 minutes on each side or until browned. Place turkey in a single layer over onion mixture in slow cooker, overlapping slightly. Add orange sections. Cover and cook on LOW for 4 hours.
3. Remove turkey from slow cooker. Remove bones from turkey; discard bones. Place turkey on a platter. Pour cooking liquid and orange sections into a medium saucepan, reserving 1/4 cup cooking liquid. Combine reserved cooking liquid and cornstarch in a small bowl, stirring with a whisk until smooth. Stir cornstarch mixture into orange mixture in saucepan. Bring to a boil; cook, stirring constantly, 1 minute or until sauce thickens.
4. Serve sauce over turkey. Sprinkle with green onions, if desired.