Step 1

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.

Step 2

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.