Make the batter. Combine the 1/2 cup flour, 1 teaspoon baking powder, 1/2 teaspoon Mexican oregano and 1/2 teaspoon ground black pepper in a large bowl. Whisk well. Add the 1 tablespoon yellow mustard and 6 ounces beer and whisk until smooth. Set aside and let rest 20 minutes.
Make the crema. Combine the 1/2 cup Mexican crema, 1/2 cup mayonnaise, 2 tablespoons lime juice, 2 chipotles, and 1/2 teaspoon salt in a blender. Blend on high until smooth.
Prepare the oil. Pour the 6 cups of oil into a Dutch oven and clip a deep-fry thermometer to the side. Heat oil to 375°F over medium heat. Line a baking sheet with paper towels.
Season the fish. Lay the fish pieces on a separate baking sheet and pat dry with paper towels. Season the fish with the 1/2 teaspoon salt and toss with the 2 tablespoons cornstarch.
Batter the fish. Using kitchen tongs, dunk a piece of fish one at a time into the batter letting the excess drip back into the bowl. Gently place in the hot oil. Don’t let go right away, let the fish fry for a couple seconds or so, lightly swishing it in the oil before letting go. This way it won’t sink directly to the bottom and stick.
Fry the fish. Fry 3-4 pieces at a time so not to crowd the pot. Cook, turning occasionally, until golden brown and crisp, about 4 minutes. Monitor the oil temperature as you are frying, adjusting it up or down to maintain 375°F as much as possible. Remove the fish to the paper towel-lined baking sheet as they finish cooking.
Serve. Place a piece of fish into each warm tortilla and serve with the crema, pico de gallo, shredded cabbage, lime wedges, and hot sauce for everyone to add to their liking.