Prepare the Fish: Pat the fish fillets dry. Set aside.
Prepare the Chips: Peel the potatoes and cut them into thick, evenly-sized chips. Blanch them by boiling for 3-4 minutes, then drain and dry well.
Make the Batter: In a bowl, combine plain flour, cornflour, baking powder, and a pinch of salt. Gradually whisk in cold lager until the mixture is smooth and slightly thick.
First Fry for Chips: Heat oil to 140°C (285°F) and fry the chips for 4-5 minutes until soft but not colored. Remove and drain.
Fry the Fish: Heat the oil to 180°C (350°F). Dip the fish fillets in the batter, shaking off the excess, and fry for 6-8 minutes until golden and crispy. Drain on paper towels.
Second Fry for Chips: Increase oil temperature to 180°C (350°F) and fry the chips again for 2-3 minutes until golden and crispy. Drain.
Serve: Plate the fish and chips together, sprinkle with sea salt, drizzle malt vinegar, and serve with tartare sauce and lemon wedges on the side.