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Heat 1 tablespoon (15 ml) of oil in a large pot over medium heat. Peel 1 small onion and 2 cloves of garlic. Use a sharp knife to chop the onion and mince the garlic into small pieces. Add the onion and garlic to the pot and stir them as they sauté. Cook the mixture for about 5 minutes, so the onion softens a little. Cut 1 pound (453 g) of pork butt or shoulder into large 2-inch (5-cm) chunks. Add the pork to the pot with the onion mixture. Stir the pork and cook it for about 5 minutes. The pork chunks should become browned, but they won't be completely cooked inside. Pour 2 cups (473 ml) of the water into the pot and stir in 1/2 cup (120 ml) of soy sauce, ¼ cup (60 ml) of oyster sauce, 3 tablespoons (37.5 g) of sugar, and 2 star anise. Heat the mixture until the liquid starts to boil. Turn the heat down to low, so the liquid bubbles gently. If there's any scum floating to the top, you can use a spoon to scoop it off and remove it. Put the lid on the pot and simmer the mixture until the meat is completely cooked and tender. This should take about 1 hour. Lift the cooked pork out of the pot and set it on a cutting board to cool a little. Once it's cool enough to handle, take two forks and shred the meat apart. Pour the liquid from the pot into a measuring jug. Pour 1/2 cup (120 ml) of the liquid back into the large pot. Pour 1 cup (240 ml) of the liquid into a small saucepan and set it aside. Stir the shredded pork into the large pot and turn the heat on to medium-high. The liquid should come to a boil. Whisk one tablespoon (9 g) of cornstarch with the remaining 1/4 cup (60 ml) of water in a small bowl. Pour half of this mixture into the pot with the shredded meat and stir it for one to two minutes. The meat sauce should thicken a little. Turn off the heat. Turn the small saucepan on to medium heat and stir in the other half of the dissolved cornstarch. Keep cooking and stirring until the liquid comes to a boil and thickens. This should take two to three minutes. This thickened sauce is the dipping sauce that you can serve with the siopao.

Summary:
Heat the oil and cut the onions and garlic. Sauté the onions and garlic for 5 minutes. Add and sear the pork over medium-high heat. Stir in the seasonings and water and bring it to a boil. Reduce the heat and simmer the mixture for 1 hour. Shred the pork. Heat the shredded pork in the pot. Dissolve the cornstarch and stir it into the meat. Make the siopao dipping sauce.