We don't have these often, but man, when we do I snarff them! The whole combination is great to me, but the sauce is really what makes it. I didn't list any amounts; just expand or contract to the number of tacos you need. I'd figure on least 2 or 3 tacos per person (more if you have hungry boys who like fish).
Fish fillets (talpia, cod, whatever)
white corn tortillas
finely shredded cabbage
sliced green onions (opt)
diced fresh tomatoes (opt)
sour cream (or plain yogurt)
Lightly season the fish--I usually give it a light salting. Then grill it, or bake in the oven (I can't seem to handle grilling fish) until flakey and a little browned. Warm the tortillas either in the microwave or in a frying pan with a little oil, just to keep them flexible. Shred the cabbage and, if you want, mix it with green onions and diced tomatoes.
Make the sauce with equal parts mayonnaise and sour cream (or plain yogurt)--I would figure on 1/4 cup of each for 4 servings. Add lime juice until the mixture thins a little, and add the seasonings to your taste (go easy on the cumin--it can take over).
Set out all the components--fish, broken into bits; cabbage mixture; warmed tortillas; sauce. Everyone can put their tacos together with fish at the bottom, topped by the crunchy cabbage, topped with sauce. Enjoy! I'm drooling just thinking about it.