1. Heat a wok or large frying pan over high heat. Add oil and swirl wok to coat. Cook onion, stirring often, until starting to brown, about 2 minutes. Add pork and cook, stirring as needed, until browned, about 15 minutes.
2. Mix remaining ingredients in a small bowl and add to pork, stirring to coat. Cover, reduce heat to low, and simmer until pork is tender, about 20 minutes. If meat starts to stick, add 2 tbsp. water.
Make ahead: Up to 2 days ahead, covered and chilled. Reheat over medium heat.