Wipe off pork with a damp cloth. Pour oil into a 2-inch-deep, 12-inch frying pan or a 5- to 6-quart pan and set over high heat. When oil is hot, fill pan with a single layer of pork and brown well on all sides, 8 to 10 minutes. Lift out pork as browned, place in a bowl, and add more meat to pan. When all the meat is browned and in the bowl, discard all but 1 tablespoon fat from pan.
Add onions, bell pepper, and garlic to pan. Stir often until onions are lightly browned, about 8 minutes. Stir in tomatoes (including juices), green chilies, salsa, hot chili flakes, and pepper.
Return pork and accumulated juices to pan, pushing meat down into sauce. Bring to a boil, then reduce heat to low, cover, and simmer until pork is tender when pierced, 1 1/2 to 2 hours, stirring occasionally. If sauce begins to stick, stir in a little water.
With a slotted spoon, transfer pork to a wide bowl. Skim off and discard any fat from sauce. Pour sauce over meat. Add salt to taste.