This is a home-made kosher substitute for Vietnamese (nuoc mam) or Thai (nam pla) fish sauce. To be honest it's not a great substitute, but it's acceptable if you need a kosher alternative or if you just can't find it in your neighborhood.
1 tin anchovies in oil
1/4 c olive oil
1 Tbsp toasted sesame oil
1/2 tsp dry mustard
3 Tbsp soy sauce
dash Worcestershire sauce
Warm the olive and sesame oil gently in a small saucepan over low heat. Add the anchovies and let them simmer. They will start to break apart; as they cook break them up futher with a stiff spatula. Cook 8 minutes then remove from heat and allow to cool. Whisk in the powdered mustard, followed by the soy and Worcestershire.
Pour into a blender and blend a few seconds until smooth.