Mince the shiso leaves, thinly slice the myoga ginger into rounds, and separately soak each in water. Grill both sides of the horse mackerel.
Debone the fish. Put the bones in a sauce pan, add water, then heat. Simmer for about 10 minutes to create a broth, while skimming off any scum that rises to the top.
Strain the dashi stock, and transfer to a bowl. Thoroughly chill in a chilled water bath, using a large bowl for the bath.
Mash the fish meat with a mortar and pestle. (Remove any skin or small bones.)
Add the miso and ground sesame, blend into the fish, spread the mixture onto a sheet of aluminum foil, then lightly brown in a toaster oven or grill.
Return the fish mixture to the mortar, then pour in the dashi soup stock a little at a time, while grinding with the pestle. ( Refer to Helpful Hints.) Transfer to a bowl, and chill over a cold water bath, as in Step 3.
Serve rice into individual bowls, then pour on the chilled soup from Step 6. Garnish with thinly sliced cucumber, shiso, and myoga, then serve. Add grated ginger to taste.
Story Behind this Recipe
I eat chilled miso soup often in the summertime. While it can be delicious with sashimi-grade horse mackerel, I made it with baked horse mackerel.
If you don't have a mortar and pestle, you could use a food processor. Taste and season the dashi soup stock as necessary. The miso taste should be weaker than miso soup.