If you don’t have white sauce you just don’t have a fish taco! This sauce is super easy to make. There are many variation on the sauce some using yogurt others sour cream. This I got from a lady who owned a taco cart in Baja and is by far the best version I have come across.

1/2 cup mayonnaise or if you want a healthier version substitute Vegenaise. I as fanatical as I am about “keeping it real” I just cant tell the difference.
juice from1/2 lime
unsweetened condensed milk

Add lime juice to mayonnaise stir. Stir in small amounts of condensed milk until sauce is consistency of a thin pancake batter. Not to runny but not to thick.