Put the soy sauce and mirin into a pot and turn on the heat to medium. Once it begins to boil, stop the heat and add the bonito flakes. Turn on the heat again and simmer on low heat for 1 minute. Stop the heat.
When the bonito flakes have sunk to the bottom, strain through a colander. Once cooled, put into a storage container, add the konbu, and refrigerate.
After 1 day has passed, remove the konbu. This will keep in the fridge for 2 - 3 months.
After straining the bonito flakes, wring out the excess moisture and dry-fry them in the frying pan to create furikake (rice seasoning flakes). If you would prefer to keep them moist, you don't need to toast them.
"Kamatama Udon": https://cookpad.com/en/recipes/146062-basic-kamatama-udon
"Mulukhiyah Chilled Udon": https://cookpad.com/en/recipes/146251-mulukhiya-chilled-udon-with-tempura-crumbs
Simmered Green Peppers & Bonito Flakes You can use the strained bonito flakes from this recipe so you don't waste anything (with sake as the only other additional seasoning).
"Simmered Sesame Konbu Seaweed": https://cookpad.com/en/recipes/148307-simmered-sesame-konbu