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To quick-pickle the onions, mix them with a pinch of salt and the vinegar, then set aside. Mix all the ingredients for the tartar sauce, and set aside. Both can be made up to a day ahead and kept chilled.
To make the batter, tip the flours and spices into a small bowl with a pinch of salt, then pour over the beer or fizzy water. Mix quickly until it just comes together with the consistency of double cream – don’t over-beat (a few lumps are fine). Keep chilled until needed.
Carefully heat the oil in a large wide pan like a wok, ensuring the pan is no more than two-thirds full, or heat a deep-fat fryer to 185C if you have a thermometer. The oil is hot enough when a drizzle of batter crisps in less than a minute.
Working quickly, season the fish with salt, then dust in flour before dredging through the batter. Hold the fillet above the bowl of batter to let the excess drip back into it, then carefully lower into the oil. Repeat with the second fillet. Fry the two fillets for about 4 mins, turning once, until deep golden and crisp. Carefully lift onto a plate lined with kitchen paper to drain for a minute.
To assemble, split each roll and spread both the top and bottom generously with the tartar sauce. Place lettuce on the bottom half, then add some pickled onions and a fish fillet. Top with gherkin slices, if you like, then serve straightaway.
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