To same skillet, add onion (and a splash more oil if pan is dry). Lower heat to medium and cook until soft and golden, about 6 minutes. Meanwhile, whisk together remaining ingredients to make the sauce. Add sauce to onions, then return pork and any accumulated juices to pan. Give it all a good stir, and add a splash of water if needed to thin the sauce. Cook several minutes longer, until pork is no longer pink inside.