Tsuyu (aka kakejiru) is the standard broth for Japanese soba and udon noodles. (To be quite precise, tsuyu means any sort of broth at all, but in the absence of qualifiers it is assumed to refer to soy-based noodle broth.) You can buy commercially prepared tsuyu in any Japanese supermarket, but it's very easy to prepare your own -- like this:

Ingredients

  • 8 1/3 cups dashi (but see note below)
  • 6 Tbsp soy sauce (preferably a Japanese brand, Chinese soy tastes different)
  • 2 Tbsp mirin
  • 2 Tbsp sugar
  • 2 tsp salt (or less)
Instructions

  1. Bring dashi to a boil.
  2. Add the other ingredients.
  3. Bring back boil and remove from heat.
Not too tough, now was it? The above is a Kansai-style recipe for high-quality noodles so the broth is quite thin, barbarians with Kanto-style tastes (like me) might go as far as to double everything except the dashi. Also bear in mind that purists think soba broth should be stronger than udon broth.

The above should be enough for about 4 bowls of noodles. The broth can be refrigerated and will keep up to three days. If preparing in advance, you may wish to just use 8 cups' worth of dashi powder and make concentrated tsuyu; just add in the water when reheating.

To serve as kake soba or kake udon, cook the noodles, pour hot tsuyu on top and add a dash of finely chopped green onion. Flavor with shichimi-togarashi to taste.