Make the batter: Whisk the flour, baking powder, garlic, cayenne, mustard,oregano, 1 teaspoons salt, and pepper until well blended. Stir in the beer until there are no lumps. (Batter may be made several hours ahead and refrigerated).
Trim the fish of all blood lines, skin, and bones. Cut into pieces the size and shape of your index finger. Sprinkle with a few drops of lime juice and a little salt. (If not using immediately, wrap and refrigerate). Pour the oil into a deep, wide pan to a depth of 2 inches and heat over medium-high heat to 350 degrees. I use a thermometer, or you can drop a bit of batter into the oil and it should rise up quickly surrounded by little bubbles.
Pat the fish dry with paper towels. Check the thickness of the batter by dipping in one piece of the fish. The batter should be the consistency of medium-thick pancake batter, coating the fish easily but dripping very little. Add a little beer if the batter seems too thick.
Add the fish to the batter. Using tongs or chopsticks, swish each piece to make sure it is thoroughly coated, then lift out of the batter, let it drip once, and lay the fish gently into the hot oil. Cook a few pieces at a time until they float and the batter is set but still very light in color. If a piece sticks to the bottom, leave it alone and it will release itself.
Remove the fish to a rack to drain; reserve the frying oil. (The fish can be prepared ahead to this point, cooled on a rack, and refrigerated uncovered. Cool the oil and reserve).
When you are ready to serve, reheat the oil to 350-360°F and refry the fish a few pieces at a time until crisp and golden brown.
To serve, hold a tortilla in your hand and add a spoonful of guacamole. Top with fish and squeeze lime over the whole thing. Then add the secret sauce, salsa, shredded cabbage, cilantro and onion as desired.