Place flank steak in bottom of a 5-quart electric slow cooker; top with chopped onion and next 5 ingredients.
Cover with lid; cook on high-heat setting 1 hour. Reduce heat setting to low; cook 6 to 7 hours or until meat is tender. Or cook on high-heat setting 4 to 5 hours. Remove meat, and shred with a fork. Return meat to slow cooker, and stir.
Serve shredded meat mixture with flour tortillas, low-fat sour cream, and, if desired, salsa.