Spray 3 1/2- to 4-quart slow cooker with cooking spray. In 12-inch nonstick skillet, cook beef and onion over medium-high heat 8 to 10 minutes, stirring occasionally, until beef is browned; drain. Spoon beef mixture into slow cooker. Add remaining sloppy joe ingredients except buns; stir well.
Cover; cook on Low heat setting 3 to 4 hours.
In small bowl, mix sauce ingredients. To serve, spoon 1/3 cup sloppy joe mixture on bottom half of each bun. Spread 1 tablespoon sauce on top half of each bun; place over meat.