===Ingredients===
  * 3 x 2 1/2 in. (8x6 cm) dried kelp (konbu)
  * 1 3/4 cups (450ml) musk lime (ponzu or kala-mansi) juice, or lemon juice
  * 1 3/4 cups (450ml) dark soy sauce
  * 1/3 cup (90ml) mirin
  * 1/4 cup (70ml) tamari soy sauce
  * 4 cups (40g) dried bonito flakes 

===Technique===
Heat the dried kelp over a gas flame or under a broiler (grill), then put into a bowl with all other ingredients. Refrigerate for 3 days, then strain. Can be stored for up to 1 year. (Bottled ponzu can be purchased in Japanese stores.)

Tags: Japanese

Source: http://www.bento.com/trt-basics.html
Reprinted from the book:  The Food of Japan by Takayuki Kosaki and Walter Wagner