Add oil to equal not more than halfway full in a heavy bottomed pot. Then, carefully clip a thermometer to the side of the pot, so that it does not touch the bottom. Heat to 375F over medium to high heat, then continue to reduce the heat to maintain a continual temperature of 375F.
Cut the potatoes in half lengthwise and slice into long and skinny strips about 1/2 inch wide. Place the strips in a bowl of cold water.
Whisk together the flour, salt and baking powder until well combined in a medium bowl. Slowly add the lager, whisking as you go, until the batter is the consistency of thick pancake batter. Set the batter aside for 10 minutes.
When the oil reaches 375F, dry the potato strips well with a paper towel. Slide them into the hot oil, 5 pieces at a time. Fry them, gently moving them around the oil occasionally, until they're golden brown, 5 minutes. Remove them from the oil and place them on a wire rack to drain. Season them with salt.
Cut the fish into approximately 5 inch pieces. Refresh the batter by stirring it again to make sure it's smooth. Dip one piece of fish into the batter at a time, then carefully lower it into the oil. Gently move the fish around to be sure it doesn't stick to the bottom of the pot. Fry the fish in batches of no more than 4 pieces at a time, removing each batch to the wire rack when it's golden brown, about 8 minutes.
Serve with a wedge of lemon. Enjoy fish and chips!