Mix together the chili powder, cumin, paprika, salt, oregano and red chili flakes, set aside.
Add the chopped onion and garlic to the slow cooker with the tomato paste, lime juice, and just 1-2 teaspoons of the spice mixture. Stir everything together until fully mixed.
Sprinkle the rest of the spices all over the chuck roast, patting to help it stick to the meat.
Place the meat on top of the onion mixture and set cook on low for 7–8 hours. Total cooking time will vary for different roasts.
After meat has cooked, use two forks to shred the meat, removing any large pieces of fat or gristle as you find them. (If the meat is still too tough to shred, it needs to be cooked a little longer. Cook for an additional 30–60 minutes and check it again.) Stir to mix well with the sauce. Cover and continue to cook on low for another 30–60 minutes.
Before serving stir well again to mix the meat with the sauce. Taste meat and season with more salt to taste.