Mentsuyu is a Japanese sauce, made of dark soy sauce, white cooking sugar (“san-on-to”)and mirin (this base is called “Hon Gaeshi”), to which dashi (typically ichban dashi, aka Primary Dashi) is added.
It can be used warmed, as a base for soup, or chilled, as a cold dipping sauce for cold noodles such as udon, soba and somen.
You can buy Mentsuyu in stores in bottles, either ready to use or in concentrated form. For a dipping sauce, you dilute the concentrated with water, either 2 or 3 parts water (the bottle will tell you.) For a soup base, you will likely dilute the concentrated with 4 or 5 parts water.
1/8 cup Hon Gaeshi
1/3 cup dashi
Mentsuyu is better if prepared with the dashi about half a day ahead, to let the flavours meld.
Once diluted with dashi, store in refrigerator and use up within 3 to 4 days.