Mix together all of the seasonings (chili powder, cumin, salt, paprika, oregano) and set aside.
To the crockpot, add the diced onion, garlic, tomato paste, lime juice, and 1-2 tsp of the spice mixture you made in step one. Stir everything together.
Season the chuck roast with the remaining spice mixture, patting the spices into the meat to help it stick. Make sure you season all sides of the roast.
Place the roast on top of the onion mixture in the crockpot and cook on low for 7-8 hours.
Check on the meat periodically. Once it has cooked, use two forks to shred the meat. Once it is done cooking it should shred easily, if it still feels tough allow it to cook a little longer. Once the meat is shredded, stir so the meat mixes in with the sauce and cook on low for another 30-60 minutes.
Once the meat is shredded, mixed in with the sauce, and warm it is ready to eat, enjoy!
