Preheat the oven to 200C/180C Fan/Gas 6.
Put the potatoes into a pan and just cover with water. Add a tablespoon of salt, cover with a lid, and bring to a simmer. Simmer for 10–15 minutes, until cooked (they will slide off when pierced with a knife). The size and type of potato can affect the cooking time, so keep an eye on them after 10 minutes.
Drain thoroughly in a colander, then set the colander on top of the cooking pan and cover the potatoes with a tea towel. Set aside for 10 minutes to steam dry.
Meanwhile, put the vegetable oil, dripping, or duck fat into a large roasting tin and heat in the oven for 10 minutes.
Gently toss the potatoes around a little in the colander. Tip the potatoes into the tin of hot fat and give it a shake to spread the potatoes out in the pan, but don't worry about turning them. Roast for 20 minutes.
Give the potatoes a turn and return them to the oven for a further 20–30 minutes or until golden and crunchy.
Season the potatoes with salt and pepper and serve.
