Directions
Create the Spice Rub: In a small bowl, whisk together the chili powder, cumin, paprika, kosher salt, dried oregano, and red pepper flakes.
Set aside.
Build the Flavor Base: Spread the diced onion and minced garlic across the bottom of your slow cooker.
Add the tomato paste, fresh lime juice, and just 1-2 teaspoons (10 ml / 0.3 fl oz) of your prepared spice mixture.
Stir to combine everything into a rough paste.
Season and Cook: Generously rub the remaining spice mixture all over the beef chuck roast, patting it gently to help it adhere.
Place the seasoned roast directly on top of the onion mixture in the slow cooker. Secure the lid and cook on LOW for 7-8 hours, or until the meat is fork-tender.
Shred and Soak: Using two forks, shred the meat directly in the slow cooker, discarding any large pieces of fat.
Stir the shredded beef thoroughly to coat it in the rich, flavorful juices at the bottom of the pot.
Final Simmer and Serve: Cover the slow cooker again and continue to cook on LOW for another 30-60 minutes. This allows the shredded meat to absorb all that incredible flavor.
Before serving, give it one last stir, taste, and adjust seasoning with additional salt if needed.