Tip the self-raising flour and cornflour into a bowl with lots of seasoning. Gradually whisk in the beer, adding a little at a time until you have a smooth, thick batter.
Fill a pan no more than 1/3 full with oil and heat to 180C or until a cube of bread browns in 30 seconds.
Submerge one of the fillets in the batter, then remove and carefully lower into the hot oil using metal tongs, holding the fish half out of the oil for 2-3 seconds before carefully lowering the whole fillet away from you – this allows an air bubble to form under the batter, ensuring it doesn’t sink and stick to the bottom, and also that the hot oil doesn’t splash you.
Cook for 2-3 minutes or until the batter is really crisp and golden, then drain onto kitchen paper and season with sea salt flakes. Repeat with the remaining fish fillet (keep the other one warm in a low oven).
Serve with oven chips, peas and plenty of malt vinegar to sprinkle over.