Step 1: Begin by preparing your curry mayo. In a small bowl, combine the mayonnaise, curry powder, turmeric, lemon juice (or apple cider vinegar), finely chopped fresh cilantro or chives, and a pinch of salt and black pepper. Stir well until all ingredients are thoroughly combined. Taste and adjust seasonings. Cover and refrigerate for at least 30 minutes to allow flavors to meld.
Step 2: Prepare your fish fillets. Pat them very dry with paper towels. Cut each large fillet into 2-3 burger-sized portions. Season lightly on both sides with salt and pepper. Set aside.
Step 3: Assemble your burger toppings. Slice your brioche buns in half. Wash and pat dry your lettuce leaves. Slice your red onion and tomatoes. Set aside.
Step 4: Prepare the batter. In a medium bowl, whisk together the self-rising flour, baking powder, and a generous pinch of salt and pepper. Gradually pour in the cold sparkling water (or beer), whisking until just combined. A few lumps are okay. The batter should be thick enough to coat the fish but still pourable.
Step 5: Heat the oil. In a large, heavy-bottomed pot or deep fryer, heat about 3-4 inches of vegetable oil to 350°F (175°C). Use a kitchen thermometer to ensure accuracy.
Step 6: Batter and fry the fish. One by one, dip each piece of fish into the batter, ensuring it's fully coated. Let any excess drip off. Carefully lower the battered fish into the hot oil. Fry in batches for 4-6 minutes, or until deeply golden brown and cooked through (internal temperature 145°F / 63°C). Flip halfway through.
Step 7: Remove the fish. Using a slotted spoon, carefully remove the cooked fish and place it on a wire rack set over a baking sheet lined with paper towels to drain. Season immediately with a sprinkle of sea salt. Repeat with remaining fish.
Step 8: Toast the buns. Lightly butter or brush the cut sides of your brioche buns with oil. Toast them cut-side down in a dry pan or on a griddle over medium heat for 1-2 minutes, or until golden brown.
Step 9: Assemble the burgers. Spread a generous dollop of curry mayo on both the top and bottom halves of each toasted brioche bun. On the bottom bun, layer some fresh lettuce leaves, followed by a crispy piece of battered fish. Top with sliced red onion and tomato. Place the top bun on, and serve immediately with chips (fries).