Place water and soup ingredients in large pan bringing to boil over med heat. When it boils remove from heat. Let stand a minute then strain thru muslin. Check taste, add more salt if needed.
Cook the Undo by heating 9 cups water in pan and cook for 8 minutes or according to package. Drain under cold running water. Wash off starch with your hands. Leave in sieve.
Pour soup into large pan and bring to boil. Blend the cornstarch with 4 tbsp water. Reduce soup heat to medium and gradually add the cornstarch mix. Stir constantly. Soup will thicken after a few minutes. Reduce heat to low.
Mix egg, mustard/cress and onions in small bowl. Stir soup to create whirlpool. Pour eggs slowly into soup pan.
Reheat Undon with hot water from kettle. Divide between bowls. Pour soup on top. Garnish with ginger. Serve hot.