Trim excess fat from the roast.
Combine the cumin, chili powder, salt and pepper in a small bowl. Using your fingers, rub this spice seasoning into the beef.
Place the beef into a slow cooker. Add the bay leaf, onions, stewed tomatoes, chili peppers and beef broth. Replace the cover on the slow cooker.
Set the slow cooker on low and cook for 10 - 12 hours or cook on high heat for 5 - 6 hours.
At the end of the cooking time, remove the roast from the slow cooker and place it on a cutting board. Reserve the cooking liquids in the slow cooker.
Using forks, separate and shred the beef and return it to the cooker. Heat through and keep warm until ready to serve. Immediately before serving, mix in the cilantro and scallions.
To serve, use a slotted prep spoon and place some of the beef on the bottom of the bun. Sprinkle with cheese and romaine and top with other half of bun.