In a large bowl, combine soy sauce, lime juice, olive oil, minced garlic, cumin, chili powder, black pepper, and salt. Whisk until blended. Add the flank steak and coat well. Cover and refrigerate for at least 2 hours, preferably overnight.
Transfer the marinated flank steak to your crock pot and pour any remaining marinade over the top.
Cover the crock pot and cook on low for 6-8 hours or high for 4-5 hours until tender and easily shredded.
Remove the flank steak, let it rest for a few minutes, then shred using two forks. Return the shredded beef to the crock pot and mix with juices.
Warm tortillas in a skillet or microwave until pliable.
Assemble tacos with shredded carne asada, diced onions, cilantro, and avocado slices. Serve with lime wedges.
