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.