First, sprinkle both sides of the beef (in the slow cooker) with salt, pepper, garlic powder, and oregano.
Next, add the onions, chili's, and diced tomatoes.
Now, cover and cook on low for 8 hours.
Next, shred the beef with 2 forks and add lime juice and cilantro.
Finally, place some of the shredded beef mixture on the taco and top with your favorite salsa and guacamole and enjoy.