1. Make batter: In a bowl whisk 1 cup flour, 1 tsp baking powder, ½ tsp salt. Whisk in ¾ cup cold lager and 1 egg until just combined; small lumps OK. Chill 10 min.

2. Heat oil: Bring 2 in vegetable oil in Dutch oven to 350 °F. Pat 1 lb cod cut into 4 pieces dry; season with salt & pepper. Dust lightly with flour.

3. Fry: Dip fish in batter, letting excess drip. Fry 4‑5 min total, turning once, until deep golden and 145 °F internal. Drain on rack; sprinkle with salt.

4. Toast buns & assemble: Butter brioche buns and griddle until golden. Spread bottom bun with tartar sauce, add lettuce, a fish fillet, more tartar, and a squeeze of lemon. Cap and serve immediately.