1. Put 5 cups cold filtered water into a stock pot or large saucepan.
2. Add the vinegar.
3. If using kombu (kelp), add it to the water. (Please note: kelp is not allowed on the Intro GAPS Diet. If you are on Intro, leave the kelp out.)
4. Bring to a boil, then remove the kombu and discard.
5. Add the bonito flakes.
6. Turn down the heat to low and simmer for at least 2 hours and up to 4 hours.
7. Strain through a strainer with optional cheesecloth.
8. Cool and refrigerate — or serve immediately. You can also freeze it.
You can find bonito flakes at many health food stores or Asian markets.