Heat the vegetable stock and 2 cups water in a wok or pot and let simmer for a minute. Add the mushrooms and tomatoes and cook for 2 minutes. Slowly pour in the beaten eggs and stir gently to create a web-like pattern. Season the soup with the light soy, salt and ground white pepper. Simmer for another minute, and add the cornstarch and water mixture. Cook the soup until the soup has thickened slightly, another minute. Finish the soup by adding the toasted sesame oil and garnishing with sliced scallions. Serve immediately.