Clean the mushrooms thoroughly, remove the stems, and slice them. Cut the green onion into 1-inch pieces, using only the green parts. Fry the mushrooms in oil along with the stems and green onion until the mushrooms turn golden. Drain the oil and set aside.

Mix the oyster sauce, soy sauce, and white pepper together. Stir-fry until fragrant, then add water. Add the mushrooms and mix well. Taste and adjust the seasoning as desired. Stir-fry over medium heat.