Heat oven to 200C/180Cfan/gas 6. Put the potatoes in a large pan filled with cold salted water. Bring to the boil and cook for 15 mins until a cutlery knife can be inserted into the centre of a potato with only a little resistance, then drain and steam-dry.
Arrange the potatoes in a large roasting tin, leaving enough space between each one so that they have room to spread once smashed. Using a fish slice, push down on each of the potatoes to break them up slightly. Don’t worry if some break up completely, it just means more crispy bits. Season well and drizzle over the oil. Roast for 1 hr, turning once, until golden and crispy.