In a large, heavy pot heat the oil (I use my Le Cruset). Saute the onion until just translucent, then add the minced garlic and saute another few minutes until garlic softens.
Add the green chiles and saute, stirring occasionally, for about 7 minutes, allowing most of the moisture from the chiles to evaporate.
Add the ground pork and stir well to combine. Saute until pork is fully cooked and slightly browned, about 10-14 minutes. Add the ancho powder, cumin, and oregano, stirring to coat the mixture well.
Fold in the cotija cheese and combine well. Use as tamale filling and enjoy! Top your tamales with your favorite tamale sauce (I use my Rich and Delicious Enchilada Sauce recipe for tamales, absolutely wonderful stuff).
Also works very well as an enchilada filling. Makes about 3-4 cups filling.