Finely chop the garlic, sprinkle generously with salt, then mash back and forth with the side of your knife across your cutting board until crushed to a puree. Scrape into a medium bowl and add the oregano, black pepper, mustard, base or bouillon, beer or water, and 1/2 teaspoon salt. Add the flour and baking powder to the wet ingredients and whisk just until combined.
Heat the oil in a heavy skillet to 370 degrees. While the oil is heating, cut the fish into pieces about 3 inches long by 1/2 inch square. Use a pair of tongs to pick up a piece of fish, dip it completely into the batter, and lay it into the oil. Continue with a few more pieces of fish, filling the hot oil with an uncrowded layer. Fry, turning the pieces regularly, until deep golden and crisp, about 4 minutes. Drain on paper towels and keep warm in a low oven on a wire rack set over a sheet pan while you fry the rest of the fish.
Mix together the mayonnaise, sour cream and milk. Set out with the cabbage, salsa, warm corn tortillas, limes and the crispy fish for everyone to make tacos.