Combine water and oxtail. Let boil. Continue to cook in medium heat for 15 minutes. Discard the water.
Pour 10 to 12 cups water into the cooking pot. Add the semi-boiled oxtail, onion, ginger, and star anise. Let boil.
Add Knorr beef cube. Stir. Continue to cook between low to medium heat until the oxtail becomes tender. Note: this usually takes between 2 to 3 hours. You can use a pressure cooker to make the cooking time faster. If using a pressure cooker, 35 minutes should be enough.
Add the Shiitake mushroom. Cook for 15 minutes.
Season with ground black pepper and salt. Stir.
Add scallions and bok choy. Stir.
Turn the heat off. Cover and let it stay for 5 minutes.
Transfer to a serving bowl. Serve.
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