A day or two before roasting, prepare the potatoes. You need chunks or whole potatoes roughly the size of a clementine, so peel and leave whole, halve or quarter accordingly. Place the potatoes in a large pan of cold, salted water – salting is important, so don’t skip it. Bring the water to the boil and simmer gently for about 15 mins until the potatoes are cooked all the way through but not on the brink of collapse. Gently drain the potatoes in a colander, but do not shake them or ruffle them up – just leave them to drain and cool. Once there is no more steam coming off them, place on a tray in a single layer and put them in the fridge, uncovered, until ready to roast.
Heat oven to 200C/180C/gas 6 (or put them in when you turn it up for the turkey). Pour oil into a deep roasting tin to give an even layer of about 0.5cm. Place the tin in the oven for 5 mins to heat the oil, then remove from the oven. One at a time and working methodically (starting at one corner of the roasting tin and lining them up as you go) place each potato in the oil and use a spoon to turn it so it’s completely coated. Don’t worry about the oil cooling down too much. Once all the potatoes are in the tin in a single layer, not touching, place the tin in the oven and cook for 40 mins, undisturbed.
Remove from the oven and – again, meticulously – turn each potato. Return to the oven for 20 mins, then repeat the potato-turning process again. If, at this stage, your other side dishes allow you to, turn the oven up to 220C/200C fan/gas 7 and give the potatoes a final 20 mins. By now you should have potatoes that are everything a roastie should be: deep, golden and crunchy on the outside with an extra-fluffy middle – just sprinkle with a little sea salt to serve.