Place onion and barbecue sauce in a slow cooker. Season ribs with salt and pepper.
In a large skillet, heat oil over medium-high heat. Add ribs in batches and brown on all sides, 2 to 3 minutes total. Transfer ribs to slow cooker and stand on end. Pour 1/4 cup water into skillet and cook over low heat, scraping bottom of pan with a wooden spoon to incorporate browned bits. Pour this liquid into slow cooker. Cover and cook on low heat for 8 hours.
Transfer ribs to a platter; skim fat from sauce. Pour sauce over ribs. Serve ribs with coleslaw and corn on the cob or other seasonal vegetable.