Get batter ingredients together. In a medium bowl, mix together flour, baking powder, spices, beer and egg yolk. If the mixture is too thick, feel free to add a bit more beer, but don't let it get watery -- it should be plenty thick to coat the fish. Certainly feel free to drink the rest of the can!
Cover the batter and let sit in fridge for at least 3 hours or overnight. As a variation, you can whip the residual egg white into a thick froth and fold in just before you dip fish -- no waste - makes for a lighter batter.
Heat oil to 375 or so. In a separate bowl, mix mayo, cilantro, and enough lime juice to make it "saucy". Slice the avocado and open coleslaw mix.
Cut fish into portions that will fit well in a taco after you fry them. (I use frozen tilapia fillets - cut down middle).
Once oil is hot, dip fish in batter and get well coated. Cook 1 or 2 slices at a time - drop a slice into oil while batter is still dripping off and fry for about 4-5 minutes - turning half way through. While fish is cooking, go ahead and steam the tortillas (microwave between 2 wet papertowels works).
You are done! Grab a steamed tortilla, add a pice of fish, a couple of slices of avocado, sprinkle on some cabbage mix and add sauce to taste. You can also squeeze a little lime juice just at the end.