Spray 5-quart slow cooker with cooking spray. Sprinkle both sides of beef with salt and pepper.
In slow cooker, add beef, 1/2 cup of the cooking sauce and the onion. (Reserve remaining cooking sauce in 1-quart saucepan.) Cover; cook on Low heat setting 8 to 10 hours or until beef is very tender.
Remove beef from slow cooker; shred with 2 forks. Reserve 1/2 cup beef liquid in slow cooker; discard remaining liquid. Heat reserved cooking sauce over medium-high heat to simmering. Add beef back to slow cooker, and drizzle heated sauce over beef. Spoon about 1/3 cup mixture onto each tortilla. Top each with tomatoes, cilantro leaves and lime wedge.