Heat a large saucepan over medium. Add oil, then onion. Cook, stirring constantly, until fragrant, about 1 min. Add ketchup, tomato sauce, brown sugar, hoisin, garlic powder, Worcestershire, paprika and pepper flakes. Continue stirring frequently, to keep sauce from splattering, until flavours develop, 5 to 8 min.
Preheat barbecue to medium-high. Oil grill and barbecue 4 skinless, boneless chicken breasts with lid closed. Brush chicken often with BBQ sauce until chicken feels springy and is richly glazed, about 15 min.