Mentsuyu is a soup base for Japanese noodles that is sold diluted or concentrated. The mentsuyu I use is concentrated and you should adjust the amount used if you use pre-diluted mentsuyu. It is sold in bottles and can be found in the aisle with other Japanese seasonings in most major supermarkets. Mine looks like this:

If you can’t find it, here’s a recipe I found to make your own though I have not tried it:
www.japanesecooking101.com/men-tsuyu-recipe/This recipe is adapted from
Just One CookBook.