In a pot over medium heat, heat oil. Add onions and garlic and cook until softened.
Add pork and cook, turning as needed, until lightly browned.
Add 2 cups of the water, soy sauce, oyster sauce, sugar, and star anise. Stir until well-dispersed. Bring to a boil, skimming scum that may float on top.
Lower heat, cover, and continue to cook for about 1 hour or until meat is fork-tender. Add more water in half cup increments as needed to maintain 1 1 /2 cups liquid.
With a slotted spoon, remove pork from the pot and let cool to touch. Using two forks, shred meat.
Remove about 1 cup of the braising liquid and set aside. Return shredded meat to pot and bring to a boil.
In a bowl, combine cornstarch and bout 1/4 cup water. Stir until smooth and cornstarch is dissolved. Add half of the cornstarch slurry to the pot of meat and stir to distribute. Continue to cook for about 1 to 2 minutes or until thickened. Remove from pan and allow to cool.
In a saucepan over medium heat, combine the reserved 1 cup braising liquid and the remaining half of the cornstarch slurry. Bring to a boil, stirring regularly, for about 2 to 3 minutes or until thickened. This well be the siopao sauce.