In a large bowl, whisk together beef broth, soy sauce, brown sugar, garlic, oil, vinegar, ginger, sriracha and pepper.
Place meat and onion in the slow cooker and stir in the prepared beef broth mixture.
Cover and cook on low for 7 to 8 hours or high heat, 3 to 4 hours. The meat should be very tender but retain shape.
In a small bowl, whisk together the corn starch and water. Stir the mixture into the beef broth and continue to cook for 30 minutes or until thickened.
Remove beef and shred it; place it back in the slow cooker and stir around.
Prepare your tacos by spreading a tablespoon of sour cream on each tortilla.
Add lettuce, shredded carrots, and cabbage.
Top with meat mixture and serve immediately.