Layer sliced onion and garlic in the bottom of the slow cooker. Place beef on top of onions, sprinkle with cumin, oregano, chili powder, 1 tsp salt, and add tomatoes.
Cover and cook on low until beef is tender and pulls apart easily, 7-8 hours.
When beef is done, transfer to a large bowl and shred with 2 forks. Season with additional salt to taste.
In a small bowl, combine vinaigrette ingredients, then toss with coleslaw mix and cilantro until combined.
To serve, divide shredded beef among rolls. Top with slaw, pickled jalapenos, and a dollop of guacamole.
Get creative! Try this shredded beef in tacos, burritos, taco salad, etc. Substitute chicken breast for the beef.