Prepare the fish. Pat the fish fillets dry with paper towels and season both sides lightly with salt. Set aside while preparing the batter.
Make the batter. In a mixing bowl, whisk together 1 cup of flour, cornstarch, baking powder, granulated garlic, granulated onion, paprika, salt, and black pepper. Gradually pour in the beer, whisking until a smooth batter forms. The consistency should be similar to pancake batter—thick enough to coat the fish but still pourable. Let the batter rest for about 10 minutes while heating the oil.
Heat the oil. Pour the vegetable oil into a heavy-bottomed pot or deep fryer and heat it to 350°F over medium-high heat. Use a thermometer to ensure accuracy.
Coat the fish. Place the remaining ½ cup of flour in a shallow dish. Lightly dredge each fish fillet in the flour, shaking off excess. Then, dip each fillet into the beer batter, allowing any excess to drip off.
Fry the fish. Carefully lower the coated fish fillets into the hot oil, working in batches to avoid overcrowding. Fry for 4-5 minutes, turning occasionally, until golden brown and crispy. Transfer the fried fish to a wire rack or paper towel-lined plate to drain. Immediately sprinkle with a pinch of salt.
Assemble the sandwiches. Toast the brioche buns until golden. Spread tartar sauce on both halves of each bun. Place a piece of crispy fried fish on the bottom half, followed by a mix of shredded red and green cabbage, a leaf of lettuce, and a sprinkle of Kosher salt. Top with the remaining toasted bun half.
Serve. Serve the fish sandwiches immediately while hot and crispy, with a lime wedge and extra tartar sauce on the side. Enjoy!