Simmer the water, soy sauce, salt, mushrooms and garlic until reduced by half. Add the kombu, cool, then cover and refrigerate for 24 hours.
Strain, store in fridge for up to 1 month.
Notes
Vegan Fish Sauce
So I went on a little quest to find out how to make a solid vegan substation for fish sauce, and after dipping in and out of various websites, I think I’ve got it: dried mushrooms and soy sauce, plus kombu if you have it, was the very satisfactory answer.Kombu is a type of edible Japanese kelp, most often used to make dashi. Dashi is a flavorful savory base used for lots of soups, and broths, most familiarly to many of us miso soup. If used on its own, you can make a vegetarian dashi with kombu. (If used in combination with bonito (tuna) flakes, you have a more traditional non-vegetarian dashi.) You can order kombu online, and find it in Asian grocery stores.