Clean the oyster mushrooms and tear them into bite-sized pieces.
Slice the onion and green onion and set aside.
Heat cooking oil in a pan and sauté the green onion to bring out its aroma.
Add the onion and oyster mushrooms, and stir-fry over high heat.
Season with salt and pepper to taste.
Add sesame oil, give it a final quick stir-fry, and serve.