Prepare the batter first, as it needs to rest.  In a bowl, beat the egg with a fork.  Then, add in the beer and the milk and mix together.  In a separate bowl, add the flour, baking powder, cayenne, coriander, salt and pepper.  Blend together while gradually adding the egg and beer mixture, whisking constantly until the batter is smooth. Cover with plastic wrap and set aside for a minimum of 30 minutes.  It will hold for up to 1 day in the refrigerator.   Preheat a skillet of vegetable oil at least 1" deep to a temperature of 350-375 degrees.  You should use a thermometer to ensure oil is at proper temperature.   Cut the fish into strips 1/2" thick by 4" long.  Season with salt and pepper.  Dip the fish into the batter and carefully place it in the hot oil.  Cook until golden brown and crispy.  Drain on paper towels.  Cradle a warm tortilla in your hand, add the fried fish, some lime mayonnaise, salsa, shredded cabbage, avocado slices and a fresh squeeze of lime.