Fish and Chips with Mushy Peas

Steps

1. Peel the potatoes and cut them into 3/8 inch (1 cm) thick sticks. Soak them for 30 minutes in a large bowl of cold water to rinse off surface starch, then drain and dry them very thoroughly in a clean kitchen towel: leftover water makes the oil foam and the chips go limp.
2. Pour the oil into a large heavy pot with high sides or a deep fryer, no more than halfway up, and heat it to 300°F (150°C), checking with a thermometer. Fry the chips a first time in 2 or 3 batches, 6 to 7 minutes each: they should be tender when pierced with a knife but still pale. Drain on a rack and let them rest.
3. For the mushy peas, boil them for 4 minutes in salted water, drain and return them to the pot with the butter, chopped mint, the juice of half a lemon and a pinch of salt. Crush them coarsely with a potato masher: the texture should stay rustic, with some whole peas. Keep warm, covered.
4. For the tartar sauce, finely chop the pickles, capers, half shallot and parsley. Stir them into the mayonnaise with the juice of half a lemon and the pepper. Refrigerate.
5. Cut the cod into 4 equal portions and pat them very dry with paper towels. Season lightly with salt on both sides. Put 1/3 cup (40 g) of the flour in a shallow dish, dredge each piece and tap off the excess, leaving only a thin coat.
6. Make the batter at the last minute: in a bowl, mix the remaining 1 cup (130 g) flour, the cornstarch, baking powder and a pinch of salt. Pour in the beer straight from the fridge, whisking just until smooth and as thick as heavy pancake batter; a few small lumps are fine, while overmixing makes the batter stretchy.
7. Raise the oil to 350°F (180°C). Dip a piece of fish in the batter, lift it and let it drip for 3 seconds, then lower it into the oil away from you, holding it at the surface for a second so it floats instead of sticking to the bottom. Fry 2 pieces at a time for 5 to 7 minutes, turning halfway, until the batter is golden, well puffed and sounds hollow. The fish should reach 140°F (60°C) inside and flake easily. Drain on a rack, salt right away and keep warm in a 200°F (100°C) oven with the door ajar.
8. Raise the oil to 365°F (185°C) and fry the chips a second time, in 2 batches, for 2 to 3 minutes, until golden and rigid. Drain and salt immediately, while they are still glossy with oil.
9. Plate each serving with a piece of fish, a generous pile of chips, a scoop of mushy peas, the tartar sauce and a wedge of the last lemon. Put the malt vinegar on the table for sprinkling over the chips and fish.