In a large saucepan, combine chicken stock, ginger, and garlic. Simmer for about 15 minutes.
After the 15 minutes, discard the ginger and garlic.
In a cup, thoroughly combine the cornstarch and cold water. Whisking constantly, slowly pour it into the soup and cook until the soup is slightly thickened.
Stir in the salt and pepper.
In a small bowl, whisk together the egg and vegetable oil.
Bring the soup to a very low simmer and pour the egg mixture in a large circle on the surface of the soup.
Once the egg sets, stir gently and serve. You can add the sliced green onions and cilantro for garnish.