Peel the potatoes, then cut into halves, or quarters if large. Bring a large pan of water to the boil, add a little salt, then tip the potatoes into the pan. Bring back to the boil and cook for 7 mins. Drain really well, then return to the pan and sprinkle over the flour. Place a lid on top of the pan, then pick up the pan using oven gloves or a tea towel and give it a few really hard shakes so the potatoes get bashed around (this will make them nice and crisp).
Heat oven to 190C/fan 170C/gas 5. Spoon the goose fat or sunflower oil onto a large, lipped baking tray and leave to heat on the top shelf of the oven for 5 mins. Carefully pull out the tray, make sure its surface is evenly coated with the fat, then tip on the potatoes. Roast for 20 mins, then turn up the oven to 220C/fan 200C/gas 7 and roast for another 30 mins, turning the potatoes once, until golden and crisp. Sprinkle with a little salt and serve straight away.
If freezing ahead, simply follow the recipe to the end of step 1, then let the potatoes cool. Line 2 baking trays or sheets with greaseproof paper. Arrange the potatoes on the paper, making sure none of them are touching each other, then place in the freezer. Once frozen solid, tip the potatoes into a bag to free up some freezer space. Can be frozen for up to 1 month. Cook for 30 mins at 190C/fan 170C/gas 5 before turning the oven up.