Rub spice mix on both sides of chuck roast and place in a dutch oven or slow cooker.
Place quartered onions on the sides of the meat in pot.
Add fire roasted tomatoes on each side of the beef.
Pour beef stock and lime juice on the side until it covers the bottom of the pot and the cooking liquid rises to the top of the meat. Add sweet peppers if desired. (optional)
Put a lid on the dutch oven and place it in the oven for 3 hours on 350 degrees. If using a slow cooker, set it to 8 hours.
At the end of the cooking time, uncover pot and shred meat directly in the pot by using 2 forks to pull it apart. It should pull apart easily if cooked properly.
Remove excess fat from the meat and discard. Smash the onions and stir with cooked tomatoes and beef to create a tasty tomato sauce or gravy.
Notes
There's no need to sear the meat before cooking. Searing doesn't make a big difference in the final outcome.
If you don't have some of the seasonings on hand, it's okay to leave them out. There will be plenty of flavor as long as you use most of what's recommended.