Step 1: Prepare the Cilantro Lime Crema : In a small bowl, whisk together sour cream, mayonnaise, lime juice, lime zest, cilantro, garlic, salt, and pepper. Adjust seasoning as needed. Cover and refrigerate until ready to serve. (The flavors intensify as it chills.)
Step 2: Prep the Fish : Pat the fish fillets dry with paper towels and cut into 2–3 inch strips. Season lightly with salt and pepper. In a medium bowl, combine flour, cornstarch, baking powder, garlic powder, paprika, cumin, salt, and pepper. Gradually whisk in cold sparkling water (or beer) until the batter is smooth and thick enough to coat the back of a spoon.
Step 3: Fry the Fish : In a deep skillet or pan, heat about 1 inch of oil to 350°F (175°C). Dip each piece of fish into the batter, letting the excess drip off. Carefully place into the hot oil and fry for 3–4 minutes per side, until golden brown and crispy. Remove and drain on a wire rack or paper towels. Keep warm in a 200°F (95°C) oven while you fry the rest.
Step 4: Warm the Tortillas : Warm tortillas in a dry skillet over medium heat for 15–20 seconds on each side, or wrap in foil and warm in the oven. Keep them covered with a towel to stay soft and pliable.
Step 5: Assemble the Tacos : Layer shredded cabbage on each tortilla. Add 1–2 pieces of crispy fish. Drizzle with cilantro lime crema. Top with avocado slices, diced tomatoes, and fresh cilantro. Serve immediately with lime wedges on the side.