
Place mirin and sake in a medium sized pot to let the kombu soak in the liquid ingredients to release its umami flavours.
Put the pot over medium heat, once it's boiled turn the heat down and simmer for 5 minutes over low heat.
Add bonito flakes (katsuobushi) and simmer for a further one minute. *2
Turn the heat off and let it cool down.
Pour soy sauce into the saucepan.
Using a fine-mesh strainer lined with a kitchen paper towel, strain out all solids from the mentsuyu into another clean container to keep.