Heat a frying pan on medium and add 1 tsp cooking oil. Add 100 g boneless chicken thigh to the pan and season with 1 pinch salt. Fry until cooked through and then remove the pan from the heat.
While the chicken is cooking, bring a small pot of water to a rolling boil and sprinkle with salt. Add 100 g Japanese mustard spinach (komatsuna) and boil for 30 seconds. After 30 seconds, transfer it to a bowl of ice cold water to stop the cooking process and set aside for later.
Rinse out the pot and reuse it. Add 400 ml dashi stock, ½ tsp mirin and 4 pieces cut rice cakes (kirimochi). Simmer over a low heat until mochi is soft on the outside. If the pot is small, cook them in batches in a single layer. Avoid stacking them and place cooked pieces in the serving bowls.
Once the mochi is softened, add 2 tsp Japanese soy sauce (koikuchi shoyu) along with the cooked chicken from earlier and simmer for about 1 minute.
Divide the soup into bowls. Squeeze the water out of the Japanese mustard spinach thoroughly and then place it on top of the soup along with 2 slices of 8 slices kamaboko fish cake and a sprinkle of bonito flakes (katsuobushi) per portion.