Combine all of the marinade ingredients and pour over the meat. Put in the refrigerator over night.
Place the meat in a slow cooker, and dispose of the marinade.
Add juice from 1 orange, mixed with the juice from 1 lime and a small amount of white vinegar. Then add the salt, pepper, cumin and oregano, and finally the bay leaves, the thyme, and the avocado leaves .
Cook on the high setting of your slow cooker for 3 hours or until fork tender.
Shred the meat, and return to the slow cooker
In a small pot, cook the chilies, the tomatoes, the garlic, and the onion in one cup of water (as shown in the post), blend together and pour through a strainer directly onto the shredded beef. It helps to use a wooden spoon to push down on the mixture so that as much liquid as possible comes out of the strainer. You can use some additional water if the mixture is too thick to strain.