Season the pork with salt and pepper. Brown the pork pieces in oil on all sides in a large frying pan, then place the pork pieces in the slow cooker.
Saute the chopped onion, garlic, and 1 chopped chile pepper in remaining oil on medium-high for 5 minutes. You may add more oil to the frying pan if needed. Add the onion, garlic, and chile to the slow cooker. Add chicken broth and cook on high for 4 hours.
Brush the 12 whole chile peppers with oil and place on a baking sheet. Cook under the broiler for 5 minutes on each side.
Remove pulp and seeds and cut the peppers into pieces, and place in food processor or blender with green enchilada sauce, chile powder, and cumin. Puree this mixture (you may need to do 2 batches depending on the size of the blender or food processor), then refrigerate.
When pork has cooked 4 hours, remove the pieces from the slow cooker and shred, then place back into the slow cooker. Add the pureed chile pepper mixture, stir well, and cook on medium for an additional 3-4 hours. Serve in bowls over rice or with warm flour tortillas.