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For the tartare sauce, mix the red onion with the lime juice and zest and a small pinch of salt, leave for 5 mins to soften a little, then mix in the mayonnaise and coriander. Cover and chill until needed. Can be made a day ahead.
For the batter, add the flours, cayenne and bicarb to a bowl, then whisk in the water or lager and set side. Pour the oil into a deep, wide pan ensuring it is no more than two-thirds full, or heat a deep-fat fryer to 180C if you have a thermometer. If you don’t, drizzle a little batter into the oil – it’s ready when the batter sizzles and turns crisp in less than a minute.
Put a little plain flour in a shallow dish then, working quickly, dust the fish in the flour, then dredge through the batter. Hold a nugget of fillet above the batter to let the excess drip back into the bowl, then very carefully lower into the oil. Fry as many nuggets as you can without the pan being overcrowded – you may need to do it in batches. Fry the fish for about 4-5 mins, turning once, until deep golden and crisp. Lift onto a plate lined with kitchen paper to drain for a minute, then serve in a basket with the tartare sauce for dipping.