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Scrub the potatoes, cut into 1.5cm thick lengthways slices, then cut each slice into 1.5cm thick chips. Tip the chips into a large saucepan, pour in enough water to just cover, bring to the boil, then lower the heat and gently simmer for 4 mins only. Drain, tip onto a clean tea towel, pat dry, then leave to cool. Can be done 1-2 hrs ahead. Heat oven to 220C/fan 200C/gas 7. Put a large shallow non-stick roasting tray in the oven with 1 tbsp olive oil and heat for 10 mins.
Transfer the chips to a bowl and toss in the remaining oil using your hands. Tip out in a single layer onto the hot roasting tin. Bake for 10 mins, then turn them over. Bake 5 more mins, then turn again. Bake for a final 5-8 mins until crisp. Drain on paper towels.
While the chips are in the oven, cook the peas in boiling water for 4 mins. Drain, then tip into the pan and lightly crush with the back of a fork. Mix in the oil, lemon juice and freshly grated pepper. Cover and set aside.
The fish can also be cooked while the chips are in the oven. Pat the fillets dry with paper towels. Put the 1 tbsp of flour on a plate and use to coat each fillet, patting off the excess. Pour the oil for frying into a heavy, medium non-stick wok or wokshaped pan. Heat to 200C (use a thermometer so you can check the oil stays at that temperature). Mix 50g flour, cornflour, a pinch of salt and some pepper. Lightly whisk the egg white with a balloon whisk until frothy and bubbly, but not too stiff. Pour the water into the flour mix, gently and briefly whisking as you go. The batter shouldn’t be completely smooth. Add the egg white, then lightly whisk in just to mix. Try and keep as many bubbles as you can so the batter stays light. Cooking two pieces of fish at a time, dip them in the batter to coat, let some of it drip off, then lower into the hot oil using a slotted spoon. Fry for 5-6 mins, making sure the oil stays at 200C and turning the fish over halfway through so it is golden all over. Lift out with a slotted spoon, drain on kitchen paper. Check the oil is back up to 200C, then repeat with remaining fish. Reheat the peas and serve with the fish, chips and lemon wedges.