Slice the potatoes about 1/16 of an inch thin with a vegetable slicer or mandolin. Transfer to a large bowl and soak in tepid water for 30 minutes.
In a large mixing bowl, whisk together the flour, baking powder, and the 2 teaspoons salt. Pour in the beer and whisk to form a smooth batter. Set aside.
Pat the fish very dry. Cut the fish lengthwise into 1 by 3-inch strips.
Pour enough oil into a tall heavy-bottomed pot so it comes about a third of the way up the sides. Heat the oil over medium heat until a deep frying thermometer inserted in the oil registers 360 degrees F. While the oil heats, drain and spin the potatoes dry in a salad spinner. Spread them out on a pan and pat completely dry with paper towels.
Preheat the oven to 200 degrees F. Working in small batches, fry the potatoes until golden brown, about 2 to 2 1/2 minutes. With a slotted spoon, carefully transfer the chips to a paper towel-lined pan to drain. Season with fine salt to taste. (Make sure to let the oil come back to 360 degrees before frying each batch.) Transfer the potatoes to the oven to keep warm.
Season the fish with the salt and pepper. Working in batches, dip the fish strips in the batter to coat evenly. Carefully add to the oil. Fry the fish, turning once, until golden and crisp, about 3 to 4 minutes. Transfer to a paper towel-lined pan to drain. Keep the fish warm on a heatproof platter in the oven while you fry the remaining fish. (Just like with the potatoes, make sure the oil returns to temperature before adding more fish.)
Serve the fish with the chips and malt vinegar and/or tartar sauce.
In a small mixing bowl, stir together the mayonnaise, capers, cornichons, parsley, lemon juice, tarragon, lemon zest, hot sauce, and scallion. Refrigerate for 30 minutes for the flavors to come together. Serve.
For a thicker and puffier crust on the fish, dredge the strips in flour, pat off the excess and then dip in the batter. Do not fry too many pieces of fish at once or the oil temperature will drop too low and the fish will be greasy.
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