The recipe for vegan fish sauce below is based on these ingredients and its the best one I've found so far to replace fish sauce in vegetarian/vegan recipes. If you can't find mushroom soy sauce, add a few dried shiitake to the seaweed mixture in the beginning.
Vegan Fish Sauce
(makes about 3 cups)
1 1/2 cups shredded seaweed (I use wakame)
6 cups water
6 fat cloves garlic, crushed but not peeled
1 T peppercorns
1 cup mushroom soy sauce
1 T miso
Combine wakame, garlic, peppercorns and water in a large sauce pan and bring to a boil. Lower heat and simmer about 20 minutes. Strain and return the liquid back to the pot. Add soy sauce, bring back to a boil and cook until mixture is reduced and almost unbearable salty. Remove from heat and stir in miso.
Decant into a bottle and keep in the refrigerator. Use one-for-one to replace fish sauce in vegan recipes.
Note: You should be able to find wakame in most health food and Japanese grocery stores. Its also readily available online.
(Image: Dana Velden)