Salt and pepper the pork. Preheat oven to 250 F
Heat vegetable oil in a medium heavy pot (Dutch oven) on medium high. Add the pork, and brown on all sides - about 10-12 minutes total. Pour off excess fat.
Add the 1/2 of the jar of Mexican sauce and chicken stock so that at least half of the pork is covered by the liquid. If you need more liquid, add water. Bring to a boil, cover, and transfer to the 250 F oven.
Braise the pork until very tender and shreds easily, about 1.5 to 2 hours. Skim fat from sauce, and transfer pork to a large bowl. Let cool slightly, then shred with 2 forks. Place pork back into the braising liquid (this works best if the pork rests in the liquid overnight - can be prepared up to 3 days ahead).
Preheat oven to 425. Lay out the tortillas on a baking sheet and spray or brush lightly on both sides with oil; stack them in twos. Slide the tortillas into the oven and bake just long enough to make them soft and pliable, about 3 minutes. Remove from the oven and stack them in a single pile; cover with a kitchen towel to keep warm.
Remove any visible fat from the shredded pork. Spread some of the remaining sauce over the bottom of a 13x9-inch baking dish. Lay out a warm tortilla, top with a portion of meat and some cheese, and roll up. Lay in the baking dish, then continue filling and rolling tortillas until all are used. Spread the remaining sauce over the enchiladas and sprinkle with cheese.
Bake 10 to 15 minutes, until the cheese is starting to brown (the meat should be warm by then). Sprinkle with onion and cilantro sprigs, serve with some sour cream, avocado, and lime wedges, and you’re ready for dinner.