Arrange onion in single layer at bottom of slow cooker crock and top with steak.
In a small bowl, stir together juice, oil, vinegar, garlic, chili powder, cumin, paprika, salt, pepper, smoked paprika and ancho chili pepper. Pour over steak
Cover slow cooker and cook on HIGH for 2 to 3 hours or LOW for 5 to 6 hours.
Remove meat to a cutting board. Using 2 forks, shred meat. Reserve juices. Transfer shredded meat back to crock and spoon juices as desired over meat to moisten. Set slow cooker to WARM to keep meat warm for serving.