1. Put the oil in a large, heavy skillet over medium-high heat. Combine the beer, egg, and flour in a medium-size mixing bowl. Stir in the mustard, cayenne, salt, and several grindings of black pepper. Put 6 pieces of fish in the batter, coat well, and add to the skillet. Cook for 3 minutes, turn the pieces, and cook for 3 more minutes, or until golden brown. Drain on a paper towel-lined platter. Repeat with the remaining 6 pieces. Reduce the heat if needed to prevent burning.
2. While the fish cooks, peel and quarter the onion and remove the leaves from the cilantro sprigs. Put the onion and cilantro in a food processor and pulse several times, or until coarsely chopped. Add the mayonnaise and yogurt tot he processor and pulse just once or twice to combine. Put the onion-mayonnaise sauce in a small bowl.
3. Halve the cabbage lengthwise and remove the core from each half. Place each half, flat side down, on a cutting surface and cut into thin shreds with a chef's knife. (Or shred using the large holes of a four-sided grater or the grating attachment of a food processor.) You should have about 4 cups. Put the cabbage in a small bowl.
4. Halve, pit, and peel the avocados. Cut each half lengthwise into six slices. Lay the slices on a small plate. Cut the lime into 4 wedges and put in a small dish. Put the salsa in a small bowl with a slotted spoon (so you won't get a lot of liquid in your tacos).
5. Bring all the ingredients for the tacos to the table and allow diners to make tacos one at a time as follows: Put a few tablespoons cabbage in the middle of each tortilla. Top with 1 piece of fish, 1 tablespoon of the onion-mayonnaise sauce, 1 tablespoon salsa, 2 avocado slices, and a squeeze of lime. Fold and eat.
This recipe is from the book Cooking to Beat the Clock
by Sam Gugino.