Trim chuck tender roast of visible fat. Sprinkle evenly with salt, pepper and garlic powder; gently rub into meat.
For optional browning, heat 1 tablespoon oil in large nonstick skillet over medium heat until hot. Brown on all sides, approximately 2 minutes per side.
Add roast to 3 1/2 to 5 quart slow cooker. Cover beef with barbecue sauce. Add onion. Cover and cook on LOW 9 to 10 hours or on HIGH 5 to 6 hours or until roast is fork-tender.
Remove roast and onions from slow cooker. Shred beef with 2 forks. If desired, skim fat from cooking liquid, transfer to a saucepan and reduce by half. Serve alongside beef.