Peel the potatoes, then halve or quarter them (depending on size). Bring a large pan of salted water to the boil and add the potatoes. Cook for 5 minutes then drain well. Return the potatoes to the pan, sprinkle over the flour and mustard powder, put the lid on and shake a couple of times to lightly crush the potatoes and coat them.
Put the roasting tray with the oil and butter into the oven (already preheated to Gas Mark 5, 190°C, fan 170°C, for cooking the turkey). When the oil is bubbling, carefully add the potatoes, roll them in the oil, and roast for 20 minutes.
You can make the roast potatoes up to 1 day in advance.
See more Winter vegetable recipes