Lightly grease slow cooker bottom and sides with vegetable oil. Layer ribs and onion in slow cooker. Combine remaining ingredients, stirring till smooth. Pour over ribs. Cook on low for 8 hours. When time is up, remove ribs and set aside. Use a metal spoon to skim off the grease from the top of the sauce. This is an important step. When all the grease is removed, return ribs to slow cooker and gently stir to coat them in sauce (the meat will be falling off the bone at this point).