To make dashi (stock): Rinse kelp and pat dry. Place kelp in a medium pot, along with the water. Steep in the water for 3 hours.
After steeping, bring the water with kelp in it to the boil. Remove and discard the kelp. Remove pot from the heat to cool for 5 minutes.
Place the fish flakes in the pot of water. Bring to the boil. When the shavings have sunk to the bottom of the pot, strain, reserving the liquid. This is the dashi, the basic stock for many Japanese soups and dishes.
While making the dashi, place the spinach in a small pot of water and bring to the boil. When the spinach is cooked, drain well.
Pour salt, soy sauce and sake in the heated dashi. Ladle the soup into bowls. Into each bowl place one piece each of mochi cake, spinach and fish paste. Put a piece of yuzu peel in each bowl.
If you can't find yuzu, an East Asian citrus fruit, use a mandarin or grapefruit. It is said that the flavour of the yuzu resembles both.