Whisk soy sauce, barbeque sauce, hot pepper paste, sugar, ginger root, red pepper flakes, sesame seeds, and black pepper together in a small bowl.
Place pork in a large bowl. Pour soy sauce mixture over pork; toss to coat. Marinate in the refrigerator, at least 3 hours and up to overnight.
Preheat a large skillet or grill pan over medium-high heat. Add pork, discarding marinade. Cook, stirring frequently, until pork begins to brown, about 5 minutes. Stir in onion; cook until soft, about 10 minutes.