Heat the saucepan and pour-in the beef stock. Bring to a boil.

Add the onions, garlic, soy sauce, Worcestershire sauce, brown sugar, salt, ground black pepper, and star anise then simmer for 8 to 10 minutes. Make sure to cover the saucepan to trap the steam. Add extra water as needed.

Turn of the heat then filter the liquid using a fine-mesh kitchen strainer. This is done by pouring the simmered mixture on a strainer with an empty bowl below to hold the liquid.

Heat a clean saucepan and pour-in the filtered mixture. Bring to a boil.

Add the cornstarch diluted in water then stir continuously until the texture becomes thick.

Turn off the heat and allow the mixture to cool down then transfer the Siopao sauce to a serving container.