Cut the potatoes into quarters, or eighths if really large – you want to end up with even-size 5cm pieces. Put the potatoes in a large pan of salted water and boil for 5 mins, just until the outside of the potato starts to soften. To check, try scraping a potato with a fork – it should be easy to make a mark but you shouldn’t be able to slide it into the flesh. Drain really well.
Return to the pan and scatter over the flour and polenta (if using) and a little salt. Place a lid on top of the pan and give it a couple of good shakes, coating the potatoes in the flour and polenta and lightly crushing the sides.
Heat oven to 190C/170C fan/gas 5. Place the goose fat and oil in a roasting tin large enough to hold all the potatoes in one layer and heat in the oven for 5 mins. Quickly tip the potatoes in and return to the oven. Cook for 30 mins, turning once, then increase the heat to 220C/200C fan/gas 7. Cook for 20 mins more or until crisp all over. Sprinkle with a little more salt, then serve.
The roasties can be boiled and coated in flour up to 1 day ahead. Keep covered in the fridge until ready to roast.