Heat oven to its hottest setting, ideally 250c/fan 230c/gas 9. Cut each potato into 1cm- wide slices, then into 1cm-thick sticks. Blanch in a large pan of boiling water for 3 mins, and then drain well.
Shake the potatoes out onto a roasting pan, drizzle with oil, ensuring each chip is evenly coated, then season well. Bake for about 15-20 mins, turning the potatoes at least twice, until golden brown and crisp