Preparation

• Make the sauce: Mix all the sauce ingredients in a medium bowl; refrigerate for up to 5 days.
• If using fresh fish: Fill a large, deep, heavy pot with vegetable oil to about 11/4 inches deep. Heat the oil to about 350°F. (To check the oil temperature, use a deep-fry thermometer, or add a cube of bread to the oil, which should bubble immediately.)
• Meanwhile, prepare the batter: Mix the flour and 1 teaspoon salt in a medium bowl and stir in the beer until smooth. Gently fold in the beaten egg whites.
• Season the fish with salt. To cook the fish, work in batches of about three or four pieces at a time. Using kitchen tongs, dip each piece in the batter, let any excess drain off, carefully submerge the fish in the hot oil, and fry until golden brown and cooked through, about 5 minutes. Transfer to a baking sheet lined with paper towels.
• If using frozen batter-fried fish fillets: Bake them according to the package instructions.
• To serve: Just after the fish comes out of the fryer (or oven), arrange in a heated dish on the table. Next to it set the tartar sauce, hot tortillas, shredded cabbage, lime quarters, and pico de gallo so each person can assemble his or her own tacos.