In a saucepan, combine the sake, mirin, and soy sauce.
60 ml sake, 135 ml mirin, 120 ml light soy sauce
Add the kombu and katsuobushi.
0.5 piece kombu, 5 g katsuobushi
Bring to a gentle boil over medium-low heat.
Reduce the heat to low and simmer for 5 minutes. Remove from the heat and let cool.
Strain through a fine-mesh sieve (save the kombu and katsuobushi for another use; see below).
Notes
How to Use Mentsuyu
This Mentsuyu is concentrated. As a general guide, use a 1:2 mentsuyu-to-water ratio for Zaru Soba/Udon dipping sauce, and 1:4 for Udon/Soba noodle soup. Adjust to taste by adding more mentsuyu or water.
Storing Homemade Mentsuyu
Store Mentsuyu in a Mason jar in the refrigerator for up to one month.
What to Do with Used Katsuobushi and Kombu?
Place the used kombu and katsuobushi in an airtight container and refrigerate for up to one week, or freeze for up to one month. Reuse the spent katsuobushi and kombu to make homemade furikake.