1. Place onion in a 3- to 3 1/2-quart electric slow cooker coated with cooking spray.
2. Heat a large nonstick skillet over medium-high heat; coat pan with cooking spray. Add steak; cook 3 minutes on each side or until browned. Place steak over onion in cooker; pour tomatoes and beer over steak. Cover and cook on LOW for 8 hours or until steak is very tender.
3. Shred steak with 2 forks in slow cooker; stir in molasses and salt. Let steak stand 10 minutes before serving.