In a medium bowl, combine all of the dry rub ingredients. Stir with a fork to combine well. Add steak and rub both sides, pressing the mixture into the meat. If you have time, marinate overnight in the fridge, not necessary if pressed for time.
In a small food processor, add the 'paste' marinade ingredients and pulse until a thick paste is formed. It’s okay if there are still chunks of the ingredients visible. Rub both sides of the steak with this paste and place meat into a slow cooker.
Top steak with remaining onion, tomatoes, lime juice and bell pepper. Cook for 4-6 hours on high or 8 hours on low.
Serve with corn tortillas, salsa, guacamole, pickled vegetables or other desired toppings.