Prepare the Chips: Peel the potatoes and cut them into thick chips, about 1.5cm wide. Place them in a large bowl of cold water and rinse them two or three times to wash away the excess starch. This is a crucial step for crispy chips! Drain them thoroughly and pat them completely dry with a clean tea towel or kitchen paper.

First Fry (Blanching the Chips): Heat the oil in a deep-fat fryer or a large, heavy-bottomed saucepan to 130°C (use a cooking thermometer for accuracy). Carefully lower half of the chips into the oil and cook for 7-8 minutes. They should be soft but not coloured. I find that using a spider strainer or slotted spoon is the safest way to do this. Remove the chips and let them drain on a wire rack. Repeat with the remaining chips. At this point, the chips can be set aside for a couple of hours if you're preparing ahead.

Make the Batter: While the chips are resting, prepare the batter. In a large mixing bowl, whisk together the plain flour, cornflour, baking powder, salt, and pepper. Just before you're ready to fry the fish, pour in the very cold beer or sparkling water and whisk lightly until you have a thick batter, about the consistency of double cream. It's okay if there are a few small lumps – don't overmix it.

Prepare the Fish: Pat the fish fillets completely dry with kitchen paper. Set up a shallow dish with a little plain flour for dusting. Season the fish fillets lightly with salt and pepper, then dust each one in the flour, shaking off any excess. This helps the batter adhere properly.

Fry the Fish: Increase the oil temperature to 180°C. Dip a floured fish fillet into the batter, ensuring it's fully coated. Let any excess batter drip off, then very carefully lower the fish into the hot oil, holding one end and laying it away from you to prevent splashing. Fry two fillets at a time for 6-8 minutes, turning halfway through, until the batter is golden, crisp, and bubbling.

Drain the Fish: Once cooked, remove the fish from the oil and place it on the wire rack to drain. Sprinkle immediately with a little sea salt. Keep the cooked fish warm in a low oven (around 80°C) on the rack while you finish cooking.

Second Fry (Crisping the Chips): With the oil still at 180°C, return the blanched chips to the fryer in batches. Cook for another 3-4 minutes, or until they are a deep golden brown and wonderfully crisp.

Serve Immediately: Remove the chips from the oil, drain well, and sprinkle generously with sea salt. Serve at once with the hot, crispy fish, a good splash of malt vinegar, lemon wedges, and your favourite accompaniments.