Cut one onion into thin wedges and spread on bottom of a large slow cooker.

Add minced garlic, then top with beef.

In a medium bowl, combine the broth, Worcestershire sauce, mustard, thyme, salt, and cayenne pepper.

Pour over beef, cover, and cook on low for 11-12 hours. You can also cook it on high for 5-6 hours.

After it's done cooking, remove beef from cooker and shred beef with two forks.

Add some of the remaining juice to beef to moisten, and set aside.

Dice remaining onion into small pieces. Add to a large saucepan along with olive oil over medium heat until translucent, about 4-5 minutes.

Add shredded beef, undrained tomatoes, cumin, chili powder, and salt. Bring to a boil and reduce heat. Simmer uncovered for 5 minutes or until heated through. Stir in cilantro.

Serve on toasted buns or rolls and topped with cheese. Enjoy!