Heat the vegetable oil in a large frying pan over medium-high heat. Fry the onion and serrano pepper for 2-3 minutes until the onion is translucent. Add the garlic, tomatoes, enchilada sauce, chicken broth, tomato purée, brown sugar and ground cumin. Let the sauce cook over medium heat, stirring occasionally until boiling; season with salt, to taste. Add the shredded pork and let cook for 5 more minutes or until the meat is warm. Remove from heat and let cool slightly. This pork filling can be prepared a day ahead of time and stored in the refrigerator until you’re ready to bake the empanadas.