Finely julienne the kombu. Gently squeeze out the bonito flakes, then chop.
Put the ingredients from Step 1 in a frying pan, add the seasoning ingredients, stir, then heat on medium.
Continuously mix while boiling off the water, and sauté until the ingredients start to look glossy.
When it cools, transfer to a resealable plastic container or a jar, and store in the refrigerator.
Story Behind this Recipe
Since it was a waste to throw out the leftovers...
If you don't have mirin, substitute with a tablespoon sake and a tablespoon sugar. While it's best to eat it up as soon as possible, it should last about 2 weeks in the refrigerator since it's seasoned quite well. For the recipe on making dashi, see Recipe ID: 1586118.