In a large pot add 3 1/2 cups chicken broth, soy sauce, green onions, and mushrooms and bring to a boil. In a small bowl add the cornstarch and 1/2 cup reserved cornstarch and whisk until combined. Pour into the soup. Reduce heat to a simmer.
Slowly stir in the beaten eggs as you are stirring the soup. The eggs will start spreading out like ribbons in the soup. Garnish with additional chopped green onions if desired.