Heat the oven to 220°C / 200°C fan. Cut the scrubbed potatoes into chips about 1.5 cm thick. Boil in water for 5 minutes, drain and let them steam dry. Toss with the olive oil and half the salt, spread over two trays and roast for 35–45 minutes, turning halfway, until golden and tender.

Drain and finely chop the capers and gherkins. Mix with the mayonnaise, mustard, chopped parsley, pepper and the juice of half the lemon. Refrigerate the tartare sauce while cooking; cut the remaining lemon into wedges.

Cut the cod into four similar portions, check for bones and pat very dry. Put 30 g of the flour on a plate. In a separate bowl, mix the remaining 130 g flour, the cornflour and remaining salt. Shortly before frying, whisk in the cold beer just until a thick, pourable batter forms.

Heat the sunflower oil to 180°C in a thermostatically controlled fryer, following its minimum and maximum fill marks. For a suitable heavy pan, use a thermometer and never fill it more than one third full. The listed litre is a pan-frying allowance: choose a pan with enough safe headroom. Keep the area dry and never leave hot oil unattended.

Dust each dry fish portion in the reserved flour, shake off excess, then dip in batter. Lower carefully into the oil away from you. Fry in small batches for about 6–8 minutes, adjusting for thickness, until golden and the fish is cooked through. Check the thickest part: it should be opaque and separate into flakes; a clean probe should reach 75°C for at least 30 seconds. Drain on a rack and keep briefly in a warm oven while the remaining pieces cook.

Boil the frozen garden peas following their packet instructions, usually 3–5 minutes, and drain. Leave the peas whole. Serve the cod, chips and peas with the chilled tartare sauce and lemon wedges.