Preheat the oven to Gas Mark 6, 200ºC, fan 180ºC. Heat the fat in a large roasting tin until it’s hot. Remove and place on the hob over a high heat. Add the potatoes (they should sizzle when they hit the pan), shake to cover in the fat and return to the top shelf.
Cook for 25 minutes, give them another good shake and cook for a further 20 minutes. Remove from the fat with a slotted spoon and season. (Store the fat in the fridge for up to 3 months to use for other roasts.)