Put the soy sauce, mirin and konbu seaweed into a pan and bring to a boil. Add the bonito flakes. Turn off the heat, and leave to rest overnight.
Strain the liquid the next day, and transfer to a clean container. You'll make about 350 ml of dashi soy sauce, which will last for about 2 weeks in the refrigerator.
The strained off bonito flakes can be microwaved to evaporate the moisture, then pulverized in a blender to turn into furikake. If you simmer the konbu seaweed and bonito flakes with water added for a long time, you can make konbu tsukudani too.
Story Behind this Recipe
My mother learned this recipe ages ago from a TV program, and passed it down by word of mouth. Every since I've been making it regularly. You can buy dashi soy sauce, but it's hard to figure out how concentrated they are, plus they often have preservatives, so I no longer buy the market brands.
The key is to let it rest overnight before straining it. If you strain it too early, the umami of the bonito flakes won't transferred to the soy sauce. It's a waste to throw away the strained off bonito flakes, so eat the remains as a rice accompaniment or tsukudani (simmered in soy sauce). I did make tsukudani but I forgot to take a photo.