Put the chopped onion in the crockery insert of a slow cooker (3 1/2- to 5-quart size). Cut the roast into 2 pieces and place on the onion layer. Combine the cola, Worcestershire sauce, garlic powder, pepper, vinegar, and barbecue sauce. Pour half of the sauce over the roast. Cover the remaining sauce and refrigerate.
Cover the slow cooker; cook the beef on LOW for 9 to 11 hours, or until the roast is tender enough to shred easily. Remove the beef from the slow cooker and discard juices.
Shred the meat with two forks. Return to the slow cooker along with the remaining reserved sauce. Cover and cook on HIGH for 10 minutes, until hot and bubbly.
Serve on toasted sandwich buns with your favorite salads, slaw, and baked beans.
Nutritional information based on 8 servings:
Calories per serving: 380
Fat per serving: 10.7