1In 12-inch nonstick skillet, heat oil over medium-high heat. Add ribs, in batches if necessary; cook 6 to 8 minutes, turning occasionally, until brown on all sides.
2Spray 4- to 5-quart slow cooker with cooking spray. Place onion in cooker. Top with ribs. Cover; cook on Low heat setting 8 hours.
3In 2-quart saucepan, cook remaining ingredients over low heat 15 to 20 minutes, stirring frequently, until sauce has thickened.
4Drain excess liquid from cooker. Pour sauce over ribs. Increase heat setting to High. Cover; cook about 1 hour longer or until meat begins to separate from bones.