maangchi asked me to post my take on this fish sauce substitute, hope you can make use of it. (:
slice a medium-sized onion in half. add it to a pot with a kelp leaf about the size of your palm and some dried shiitake mushrooms in 2 cups cold water. turn off the heat once you bring the water to a boil. once it’s cooled down, strain the liquid into a container. you can throw away the onion and kelp, but definitely save the reconstituted mushrooms for another dish. add soy sauce to the liquid until it tastes as salty as fish sauce (meaning REALLY salty). if you don’t want the sauce to discolour your dish, use sea salt instead of soy sauce.
if you want the 'fish sauce' to be gluten-free, just use sea salt or gluten-free soy sauce in place of regular soy sauce.