In a medium saucepan over high heat, add the mushrooms, chicken stock, rice wine and ginger and bring to a boil. Reduce the heat to a simmer and stir in the salt and pepper.
Add the cornstarch mixture to the simmering soup and stir until the soup has slightly thickened (enough to coat the back of a spoon).
In a small bowl, whisk the egg lightly with a fork. Slowly pour the egg into the soup in a steady stream while continuouslly stirring with a ladle. The egg should cook immediately Turn off the heat to prevent from overcooking the egg. Sprinkle the green onions on top and serve.