1. Make the instant fish stock by pouring scant 1 1/2 pints (800 ml) boiling hot water from the kettle into a saucepan containing the dashinomoto cube. Set 1 3/4 pints (1 litre) cold water mixed with the vinegar to heat in another saucepan.
2. Select spinach leaves and stalks that are young and tender. Wash 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.
3. Cut the lemon rind into very fine strips. 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.
4. Divide the spinach into 4 portions and place beside the egg in each bowl. Stir the soy sauce into the hot stock and ladle into the bowls. Sprinkle the lemon rind strips on top.