Add 2 cups water to a saucepan and bring to a boil.

While the water is heating, add the chiles to a non-stick pan over medium heat. Press the chiles into the pan so they toast, turning often, don’t burn. 

Transfer the peppers to the boiling water and add the garlic, cilantro, tomato, agave and salt. Turn down to a simmer for 1 minute. Turn off the heat and steep for 15 minutes to soften the peppers.

When the peppers have softened, transfer to a food processor or blender and blend until smooth. Strain the sauce into the slow cooker. Discard the paste from the strainer.

Turn the slow cooker on low. 

Season the beef with the salt, oregano and cumin, rub the seasoning all over the meat. Add the beef to the slow cooker and spoon a little of the sauce over the meat. 

Cover and cook for 8 hours on low or 4 hours on high.

When the beef is done, remove some of the liquid until there is only enough left in the slow cooker to come half way up the meat. ** see note

Use 2 forks to shred to the beef, mixing into the liquid to moisten and flavor the meat, add more liquid if required. Taste the meat and add more salt if needed, mix well.

Serve in corn tortillas and top with chopped red onion, cilantro and any other toppings you like.