1. Fill the heavy-bottomed pot not more than halfway full of oil, and carefully clip the thermometer to the side so that it does not touch the bottom of the pot. Heat to 375F over medium-high heat, then reduce the heat to maintain a temperature of 375F.

2. Cut the potatoes in half lengthwise and slice into long, skinny 1 x 1/4-inch strips. Put the strips into a bowl of cold water.

3. In a medium bowl, whisk together the flours, salt and baking powder until well combined. Add the lager a little at a time, whisking as you go, until the batter is the consistency of thick pancake batter. Set the batter aside for 10 minutes.

4. When the oil reaches 375F, dry the potato strips well with a kitchen towel. Slide them into the hot oil, 4 to 6 pieces at a time. Fry, moving them around the oil occasionally, until they're golden brown, 4 to 6 minutes. Remove them to a wire rack to drain and season them with salt.

5. Cut the fish into 4- to 6-inch pieces. Stir the batter again to make sure it's smooth. Dip one piece of fish into the batter, then carefully lower it into the oil, letting it fall away from you. Move the fish around to be sure it doesn't stick to the bottom of the pot. Fry the fish in batches of 2 to 4 pieces, removing each batch to the wire rack when it's golden brown, 6 to 8 minutes.

6. Arrange a few pieces of fish and a few fries on each plate, and serve with a wedge of lemon.