Hon Gaeshi is a sauce used in Japanese cooking as a base for sauces and broths, such as Mentsuyu.
It is a simple sauce, made from dark soy sauce, white cooking sugar (“san-on-to”) and mirin.
People in the Kansai area of Japan add a bit more sugar to make it a bit sweeter.
To make 6 cups (48 oz / 1.4 ml):
1 litre bottle (4 1/4 cups) soy sauce
180 ml (3/4 cup) hon mirin
150g (3/4 cup) white sugar (superfine aka caster is best)
Simmer the mirin a bit in a pan (don’t let it boil, and skim any scum off), add sugar and stir till dissolved, then add the soy sauce.
This will will keep for months stored in the fridge in a tightly sealed bottle or container.
To make it into Mentsuyu sauce, add dashi.