Place the onion in a 3 to 3¼ quart slow cooker coated with the cooking spray.
Heat a large nonstick skillet over medium-high heat and coat the pan with cooking spray. Add the steak and cook until well browned on each side, about 3 minutes. Place the steak over the onion in the slowcooker and pour the tomatoes and beer over steak. Cover and cook on low for 8 hours or until the steak is very tender.
Shred steak with two forks in slow cooker, and stir in the molasses and salt. Let the steak stand for 10 minutes before serving.