Fish and Chips

Peel the potatoes and cut them into chips about 1 cm thick. Rinse under cold water until the water runs mostly clear, then soak for 10 minutes and dry very thoroughly with clean towels.

Mix the commercially prepared mayonnaise, cornichon, capers, 1 tablespoon lemon juice, and 1/4 teaspoon salt in a bowl. Using commercial mayonnaise avoids raw-egg risk; do not substitute homemade mayonnaise unless it is made with pasteurized eggs. The tartar sauce can be made up to 3 days ahead; cover and keep it chilled while you cook the fish and chips.

Heat the oil to 150°C in a deep fryer or large heavy saucepan. Fry the potatoes in two or three batches for 5 to 6 minutes, until tender but barely colored. Drain on a rack and season each batch with 1/2 teaspoon salt total.

Return the oil to 180°C before frying the potato batches in two or three batches for 5 to 6 minutes, until crisp and golden. Drain each batch fully on a rack and keep the batches separate until serving; season each batch with 1/2 teaspoon salt total.

Whisk 140 g of the flour with the cornstarch, baking powder, 1 teaspoon salt, and the pepper. Pour in the chilled lager and whisk just until combined into a smooth, pourable batter; do not overmix. Raise the oil to 180°C. Dust each fish portion with the remaining 40 g flour, shake off the excess, then dip each piece into the batter, turning to coat it completely; the batter should cling in an even layer like thick cream, not run off or form a heavy paste. For large portions, let excess batter drip back into the bowl for a few seconds before lowering the fish carefully into the oil. Fry in batches for 5 to 7 minutes, turning once, until deeply golden; the thickest part should be opaque and flake easily throughout, and reach 63°C/145°F. Rest the fish for 2 minutes before serving.

Return the potato batches to the 180°C oil for 2 to 3 minutes, frying until crisp and golden. Drain on a rack, then immediately toss or turn the hot chips with the remaining 3/4 teaspoon salt so it adheres evenly.

Arrange the hot chips and fish on warmed plates. Add the chilled tartar sauce and lemon wedges, then serve immediately with malt vinegar if desired.