Make the fish stock by pouring 3 1/2 cups boiling water from the kettle into a saucepan containing the dashinomoto cube.
Set 1 quart cold water mixed with the vinegar to heat in another saucepan.
Wash the spinach and add to a large saucepan of boiling salted water to blanch for only 30 seconds; drain in a sieve and immediately refresh by rinsing under running cold water.
Shred the lemon rind.
When the vinegar water comes to a very gentle boil, break the eggs on to a saucer one at a time and slide them into the water at the point where the water is actually bubbling; once the whites have set, remove them carefully with a slotted spoon or ladle and place each in a heated soup bowl.
Divide the blanched spinach into 4 portions and place beside the egg in each bowl.
Stir the soy sauce into the boiling hot stock and ladle into the bowls.