Spray 3- to 3 1/2-quart slow cooker with cooking spray. Cut beef into 3-inch pieces. Sprinkle beef with salt and pepper.
Mix coleslaw mix, onion, rice, barbecue sauce and water in large bowl. Layer beef and coleslaw mixture in slow cooker.
Cover and cook on Low heat setting 8 to 10 hours. To serve, fill buns with beef mixture. Serve with sliced red onions and additional barbecue sauce, if desired.