Mix cornflour in one fourth cup of vegetable stock or water. Heat oil, add ginger and garlic and sauté for half a minute. Add onion and continue to sauté till onion is soft and transluscent. Add tomato ketchup and tomatoes. Cook for about five minutes.
Add the remaining stock or water, salt and white pepper powder and stir. Bring the mixture to a boil. Add cornflour mixture, a little at a time, stirring continuously, till the soup reaches a thick consistency.
Add white vinegar and stir. Pour the whisked egg whites in a steady thin stream, while stirring lightly, allowing it to coagulate and come to the top.
Stir in coriander leaves reserving some for garnish. Serve hot immediately garnished with the reserved coriander leaves.