Put the whole unpeeled potatoes in a pan of water. Bring to the boil and cook for 15 mins. Drain and leave to cool. Can be done several hours ahead.
When ready to serve, strip the skins from the potatoes, then cut them into thick slices. Heat the oils together in a very large non-stick frying pan.
Add the potatoes in a single layer if there is room, and cook for 10-15 mins, tuning them frequently with a fish slice until they are golden and crispy. Sprinkle with the rosemary and flaky salt, then serve.