Batter: In a medium bowl, mix the flour, baking powder, spices, beer and egg yolk. If the mixture is too thick, add a bit more beer, but don't let it get watery. You want it thick to coat the fish.
Cover the batter and refridgerate for at least 3 hours. It can sit over night. Variation: whip up an egg white and fold it into the batter just before dipping the fish. (I've never added the egg white -- sounds interesting.).
Sauce: If you will be using the sauce for a condiment, combine all the ingredients. Use enough lime juice to make it "saucy".
Fish: Cut the fish into pieces that will fit well into the taco after they are fried. I usually cut mine about 1 inch by 1 inch by 3 inches.
Heat a good 1/2 inch of oil to about 375 degrees. Once the oil is hot, dip the fish into the batter to get it well coated, one piece at a time. Do not put too many pieces into the pan at once. Only cook a few at a time. While the batter is still thick on the fish and dripping, drop it into the oil and fry it about 4-5 minutes, turning to the other side about halfway through.
To serve: Serve with steamed tortillas. You can serve with the sauce, cabbage, lime and avocado as suggested above (put fish into taco, add avocado, cabbage, top with sauce and a squeeze of lime). Or lately we just serve with Spicy Chipotle Slaw inside the tortilla with the fish. Usually two pieces of fish per taco.