Make the sauce and cook the pork. Pour off all but about 1 Tbs. of the fat from the fry pan and return the pan to medium-high heat. Add the onion and sauté until golden, about 5 minutes. Add the vinegar and deglaze the pan, stirring to scrape up the browned bits from the pan bottom. Stir in the ketchup, brown sugar, molasses, red pepper flakes, Worcestershire sauce, mustard, salt and pepper. Cook, stirring occasionally, just until the mixture begins to bubble. Pour over the pork. Cover and cook according to the manufacturer’s instructions until the pork is very tender, 4 to 5 hours on high or 8 to 10 hours on low.