Check off each step as you cook.

1. 01Start the chips.Heat the oven to 425°F / 220°C. Put the potato chips in a saucepan of cold lightly salted water, bring to a boil, and simmer for 4–5 minutes. Drain well, let steam dry briefly, then toss with the measured chip oil and half the salt.
2. 02Roast until golden.Spread the chips on a large rimmed tray, with space between them. Roast for 35–45 minutes, turning once, until crisp outside and tender inside. Start the fish near the end of this time.
3. 03Prepare the batter.Heat frying oil in a deep heavy pot to 350°F / 175°C, keeping the pot no more than half full. Whisk batter flour, cornstarch, baking powder, remaining salt, and pepper together. Stir in sparkling water just until a thick pouring batter forms; add a small splash more if it will not coat the fish.
4. 04Coat and fry.Pat the fish dry again. Dust with the separate coating flour and shake off excess. Dip in batter, let excess drip briefly, and carefully lower into the oil away from you. Fry one or two fillets at a time for 5–7 minutes, turning carefully, until crisp and the thickest part reaches 145°F / 63°C. Drain on a rack and return the oil to temperature between batches.
5. 05Finish the peas.Meanwhile, boil peas for 3–4 minutes, drain, stir through butter, and roughly mash with a fork. Serve the fish promptly with chips, peas, lemon, and optional malt vinegar or tartar sauce.

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