While dough is resting, combine soy sauce, oyster sauce, sugar, hoisin sauce, char siu sauce, water, sesame oil, and cornstarch and mix to combine. In a wok or medium pan, add oil and onions and sauté until fragrant, about 1 minute. Add BBQ pork and the sauce mixture, then let simmer on low heat until sauce is thickened but still runny, about 1-2 minutes. Remove from heat and let cool down.

Punch the dough to flatten. On a floured surface, knead the dough a few times, roll into a cylinder shape, then cut into 6 pieces. Flatten each piece of dough with your palm, then use a rolling pin to roll the edges until you have a round disc about 5 inches wide and ½ inch thick. Place 2 tablespoon filling in the center, then pleat the dough edges up and around the filling to enclose completely. Repeat with remaining dough. Cover with a damp towel and rest for 30 minutes.

In a steamer lined with parchment paper, place buns and steam for 8 minutes. Turn off heat and let buns sit covered for another 5 minutes before opening to ensure dough is fluffy.