Pour enough of the neutral oil into a pot until it's about 4 inches deep. Heat it to 350℉.
While the oil heats, pat the pieces of cod dry with paper towels.
In a large bowl, add in the flour, cornstarch, ground mustard, salt, baking powder, paprika, garlic powder, and black pepper. Whisk the dry ingredients together until they're evenly combined. Pour in the Mexican beer. Whisk until combined and the mixture looks like thick pancake batter.
Working in batches, fully coat the cod in the beer batter. Allow any excess batter to drip off then gently drop it into the hot oil. Cook the cod for about 6-7 minutes until its cooked through, golden brown, and crispy. Remove the fried cod from the oil and place it onto a baking sheet lined with paper towels to drain. Fry the remaining pieces of cod.
Heat a pan over medium high heat. Add in a ½ tablespoon of neutral oil. Place a few of the tortillas into the pan. Cook them for about 1-2 minutes on each side until the tortillas are heated through. Repeat this process to warm the remaining tortillas.
To assemble the tacos, add a piece of fried cod, cabbage slaw, pickled red onions, a drizzle of the crema, a sprinkle of queso fresco or cotija, and cilantro leaves to each taco. Serve with lime wedges and enjoy!