Hiyajiru is originally from Miyazaki prefecture, it seems. However, there are other regions such as Saitama or Yamagata which has similar one.
For this, I didn't use miso, but usually, you would use miso. I simply put powdered dried sardine and pour cold water, dissolved it.
Added sliced cucumber and myoga ( sort of ginger ) and grated regular ginger on the top. The bottom is cold white rice. ( I microwaved the rice and then when added soup, put several ice cubes. )
Soothing texture is perfect for Summer morning.