Photograph: Tara Fisher
Cut the potatoes into large, even-sized pieces and put in a large pan of cold salted water. Cover and bring to the boil. Simmer for 5 minutes, then drain and return to the pan. Cover with a clean tea towel and the lid. Leave to steam dry for 5 minutes. Remove the tea towel and replace the lid. Shake the pan to roughen the edges of the potatoes, then tip on to a large baking tray, spread them out and leave to cool completely.
To freeze: open freeze the potatoes on the baking tray, loosely covered with clingfilm, until solid. When frozen, tip the cold potatoes into large freezer bags and return to the freezer.
To roast: there's no need to defrost the potatoes first. Preheat the oven to 220°C, fan 200°C, gas 7. Heat the olive oil or duck fat in a large roasting tin in the oven for 5 minutes. Carefully add the frozen parboiled potatoes and turn them to coat well in the hot oil. Cook for 40-50 minutes, turning in the oil once, until golden brown and beautifully crisp. Take out of the oven and sprinkle with crushed sea salt flakes and a little chopped rosemary, if you like, to serve.
Get ahead: freeze up to a month ahead. Parboiling potatoes before freezing them is not only a great way to get ahead, it also makes the potatoes extra crunchy when cooked. Roast from frozen while your cooked Christmas turkey is resting.