Put all the ingredients in a pan over low-medium heat. Put the bonito flakes in a dashi 'tea bag'.
When it comes to a boil turn down the heat to low, and simmer slowly for 5 minutes. Turn off the heat and leave until cool.
When the liquid is cool, take out the kombu seaweed and bonito flakes, and transfer the liquid to a clean container. Dilute by a factor of 2 to 3 when using.
Story Behind this Recipe
Mentsuyu is convenient and delicious. I used to use storebought mentsuyu, but I wanted to make my own so I looked up recipes online. After several experiment with more or less mirin, sugar and so on, I think that this version is just like the storebought one I used to use.
It's a bother to have to strain the liquid, so I put the bonito flakes in a large dashi 'tea bag'. Dry roast the used konbu seaweed and bonito flakes with sesame seeds and turn into furikake (rice sprinkles). The mentsuyu sauce will keep for about a month in the refrigerator.