In a large bowl, whisk together all-purpose flour, rice flour (or cornstarch), baking powder, salt, and black pepper. Gradually whisk in beer or sparkling water until smooth and slightly thick, similar to a pancake batter. Let rest 5 minutes while you prepare the fish.

Prepare halibut and toppings

Pat halibut fillets dry with paper towels. Set up a plate with lettuce, tomato slices, and red onion slices for topping. Slice buns and drizzle or spread a thin layer of tartar sauce on the inside of the bottom buns if desired.

Heat oil

Pour oil into a deep, heavy-bottomed skillet or pot to a depth of about 2 inches. Heat to 350°F (175°C). If you don’t have a thermometer, drop a small batter bit to test— it should sizzle immediately.

Dredge and fry the halibut

Pat each halibut fillet dry again. Lightly dust with a little flour from the batter mix (optional for extra crisp). Dip fillets into batter, allowing excess to drip off, then carefully lower into hot oil. Fry until golden and crisp, about 3–5 minutes per side depending on thickness. Remove with slotted spoon and drain on a rack or paper towels.

Assemble the sandwich

Spread tartar sauce on the bottom bun if using. Place a lettuce leaf, a fried halibut fillet, tomato slice, and onion on top. Add a squeeze of lemon over the fish if desired, then cap with the top bun. Repeat for remaining sandwiches.

Serve

Serve immediately with extra lemon wedges and additional tartar sauce on the side.