Here is a recipe that I use and the tacos are delicious. These are supposed to be authenic but do not know for sure. All I know is that we love them.
This is the recipe:
Fish Tacos
1 cup all-purpose flour
1 teaspoon baking powder
1/2 teaspoon salt
1 egg
1 cup beer
1/2 cup plain yogurt
1/2 cup mayonnaise
1 lime, juiced
1 jalapeno pepper, minced
1 teaspoon minced capers
1/2 teaspoon dried oregano
1/2 teaspoon ground cumin
1/2 teaspoon dried dill weed
1 teaspoon ground cayenne pepper
oil for frying
1 pound cod fillets, cut into
2 to 3 ounce portions (you can use any mild white fish)
1 (12 ounce) package corn tortillas
1/2 medium head cabbage,
finely shredded
Directions
1 To make beer batter: In a large bowl, combine flour, baking powder, and salt. Blend egg and beer, then quickly stir into the flour mixture (don't worry about a few lumps).
2 To make white sauce: In a medium bowl, mix together
yogurt and mayonnaise. Gradually stir in fresh lime juice
until consistency is slightly runny. Season with jalapeno,
capers, oregano, cumin, dill, and cayenne.
3 Heat oil in deep-fryer to 375 degrees F (190 degrees C).
4 Dust fish pieces lightly with flour. Dip into beer
batter, and fry until crisp and golden brown. Drain on paper
towels. Lightly fry tortillas; not too crisp. To serve, place
fried fish in a tortilla, and top with shredded cabbage, and
white sauce.
We always put pica de gallo on top of the white sauce.