Heat up the wok and lightly brown the garlic and ginger with oil.
Throw in the mushrooms, stir for 1 minute and season with the sesame oil, oyster sauce, Shaoxing rice wine, light soy sauce, sugar and chicken powder.
Pour in the chicken stock and mushroom water, then bring the liquid to a boil.
As soon as it boils, layer the top with Chinese cabbage and lower the heat to a simmer.
Add the black moss as the final layer and simmer for 30 minutes.
Optional: For a thicker sauce, mix 2 tbsp potato starch with 1/2 cup water until well combined. Bring the heat back up and slowly pour the slurry in, stirring gently.
Serve hot with rice!