Cut 2 large russet potatoes into chips. Soak in cold water for 5 minutes, then rinse until the water runs clear.
Put the chips in a pan of salted water with 1 teaspoon of vinegar. Bring to a simmer, then simmer gently for 10-12 minutes, until cooked through but not falling apart. Drain, then chill until ready to fry.
For the tartare sauce, chop 4 cornichons, 1 teaspoon of capers, and 1 shallot. Mix with 1 handful of chopped parsley, 5 tablespoons of mayonnaise, and 1 pinch of salt.
For the batter, mix 75g of self-rising flour, 25g of cornstarch, 1 pinch of turmeric, and a little salt. Whisk in 125g of cold lager until as thick as heavy cream, leaving a few lumps.
Heat about 1 liter of sunflower oil in a deep-fat fryer or a large, deep pan to 355°F / 180°C. Fry the chips for 8-10 minutes, until golden and crisp, then drain on paper towels.
Dust 2 thick cod fillets (about 150g each) with flour and dip them in the batter. Fry for about 4 minutes, turning once, until deep golden and crisp and the fish reaches 145°F / 63°C. Serve with the chips and tartare sauce.
