Gather all necessary components: dried shiitake mushrooms, kombu seaweed, tamari, maple syrup, apple cider vinegar, and water.
Place the dried shiitake mushrooms and kombu in a saucepan with water.
Bring the liquid to a gentle boil, then reduce the heat and simmer for 20 minutes.
Remove the mushrooms and seaweed from the saucepan.
Add tamari, maple syrup, and apple cider vinegar to the infused liquid. Stir well to combine all elements.
Continue to simmer the mixture on low heat, allowing it to reduce by about one-third. This deepens its richness and concentrates the flavors.
Pour the cooled liquid through a fine-mesh sieve to remove any small particles.
Transfer the finished product to an airtight bottle. Refrigerate for up to two weeks.