1. Batter: Sift the flour and cornflour together. This will help prevent lumps.
2. Whisk the flours, beer, egg and salt in a bowl. The beer will foam up at first. Make sure there are no lumps. Cover the bowl with clingfilm and chill in the fridge for at least 30 minutes.
3. Fish: Pour the oil into a large pot. Turn the heat up to medium and bring the oil to 180°C.