Preheat the oven to 220°C / 425°F / Gas mark 7.
In a large shallow non-stick roasting tray or dish add 1 tablespoon of olive oil and heat for 10 minutes.
Scrub the potatoes and cut them 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 and bring to the boil.
Lower the heat and gently simmer for 4 minutes exactly.
Drain, tip onto a clean tea towel, pat dry and leave to cool.
Transfer the chips to a bowl and toss in the remaining oil using your hands.
Tip out in a single layer carefully onto the hot roasting tray or dish.
Cook for 10 minutes, then turn them over.
Cook for 5 more minutes, then turn again.
Place back into the oven for a final 5-8 minutes until crisp. Drain on paper towels.
While the chips are in the oven, cook the peas in boiling water for 4 minutes.
Drain, then tip into a pan and lightly crush with the back of a fork.
Mix in the oil, lemon juice and freshly grated pepper.
Cover and set aside.
Pat the fillets dry with paper towels.
Put 1 tablespoon of flour on a plate and coat each fillet, shaking off the excess flour.
Pour the oil for frying into a heavy, medium non-stick pan. Using a thermometer, heat the oil to 200°C.
Mix 50g flour, corn flour, a pinch of salt and some pepper.
Lightly whisk the egg white until frothy and bubbly, but not stiff and set aside.
Pour the water into the flour mix, and gently whisk.
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 carefully lower into the hot oil using a slotted spoon.
Fry for 5-6 minutes, making sure the oil stays at 200°C 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 at 200°C, then repeat with remaining fish.
Reheat the peas and serve with the fish, chips and lemon wedges.