First, make the pickled onions. Put the vinegar in a pan with 150ml water, the coriander seeds, peppercorns, sugar and 1 tsp salt over a medium heat, stirring until the sugar has dissolved. Scoop the onions into a bowl and carefully pour over the hot pickling liquid. Leave to cool for 15 mins, then cover and chill for at least 1 hr. Will keep chilled for up to two days.
For the tartare sauce, mix together all the ingredients, then cover and chill. Will keep chilled for up to two days.
Sift the flour and cornflour into a bowl, then stir through the salt. Make a well in the centre and whisk in the cold beer to make a smooth batter. Cover and chill for 30 mins.
Heat the oil in a deep, heavy-based pan, no more than two-thirds full, until the oil reaches 190C on a cooking thermometer. If you don’t have a thermometer, check the oil is ready by dropping a small cube of bread into the oil – it should turn brown in about 10 seconds.
Dip each piece of cod into the batter to fully coat, then gently shake off the excess. Fry the fish in batches for 4 mins per batch, then remove using a slotted spoon and drain on kitchen paper.
Warm your tortillas in a low oven or in a dry frying pan. Spread over the tartare sauce, then add the fish and top with the pickled onions and shredded lettuce, plus a dash of hot sauce, if you like. Best enjoyed with a cold beer.