Place the tomatillos, serrano pepper, jalapeno peppers, garlic, half the onion and 2 cups of water in a pot. Bring the mixture to a boil, and continue to cook for 30 minutes.
Strain the pulp out of the liquid, and reserve ¾ of the liquid.
Add the pulp mixture to a food processor.
Place the remaining onion, cilantro, salt and the reserved liquid into the food processor as well, and pulse until a smooth sauce has formed.
Add the beef butt pieces, lemon pepper, oregano, salt, pepper and garlic to a large pot, and sear for 10 minutes.
Stir in the salsa verde sauce, cumin, onion, canned chiles and cilantro, and cook for another 5 minutes.
Pour everything into a slow cooker, and cook on high for 30 minutes.
Reduce the heat to low, and cook for 3 ½ hours.
Enjoy!