I pat my fish fillets completely dry with paper towels, then season them lightly with salt on both sides. Letting them sit for ten minutes helps season them all the way through, not just on the surface. Cutting each fillet into two or three pieces also makes them easier to handle in the hot oil later.
In a large bowl, I whisk together the rice flour, half the all-purpose flour, baking powder, and salt. Then I slowly pour in the cold lager and vodka, whisking just until smooth, since overmixing makes the batter tough. I pop the bowl in the fridge for about 30 minutes so it stays cold and light.
I cut my potatoes into thick batons and rinse them well to remove extra starch, then pat them completely dry. I fry them in two stages: first at a lower temperature until soft, then again at a higher temperature until golden and crisp. This double-fry method is what gives British chips that fluffy inside and crunchy shell.
Right before frying, I toss each piece of fish in the remaining plain flour, shaking off the extra. Then I dip each piece into the chilled batter, letting any excess drip back into the bowl. This thin dusting of flour underneath is what helps the batter cling properly instead of sliding off in the oil.
I lower the battered fish gently into oil heated to around 350°F, frying just two or three pieces at a time so the temperature doesn't drop. After about four to five minutes, once it's deep golden and crisp, I lift it out with tongs and let it rest on a wire rack.
I always let the fried fish sit on the rack for a minute or two before serving, which keeps the crust crisp instead of steaming itself soft. A final light sprinkle of salt right out of the fryer makes a big difference. Then it's straight onto the plate with hot chips.