Heat oil in a pot or wok, and then fry the oyster mushrooms for around 30 seconds until slightly soft.
Pour water or stock in along with ginger slices. Bring to boil and simmer for 5-8 minutes.
Turn up the heat and pour the whisked egg in. At the same time, stir the soup with a spoon either quickly or slowly (stirring quickly yields smaller egg flowers, while slowly results in larger egg flowers). When the egg flour is formed, turn off the fire. Add sesame oil and pinch of salt, and then combine well.
Serve with chopped green onions.