Boil the potatoes for 5 mins in salted water until the outside of the potatoes starts to soften. Drain well, return to the pan and shake to fluff up the outsides – alternatively, you can scratch them with a fork. (This process will make them crispier.)
Heat oven to 180C/160C fan/gas 4. Put the goose fat (if using) and oil in a large shallow roasting tin and heat for 8-10 mins until the goose fat has melted and the mixture is really hot. Remove from the oven, carefully tip in the potatoes and stir well to coat them in the fat. Spread them out as much as the space allows and roast underneath the turkey for 45 mins, turning frequently and adjusting the positions so that they brown evenly.
When the turkey comes out of the oven, turn up the heat to 220C/200C fan/gas 7, season the potatoes with the flaky salt and roast for another 30 mins, turning halfway through the cooking time, until beautifully golden.