Use a sharp knife to cut down each potato at 1cm intervals but not all the way through, then turn, and cut the other way so each potato ends with a cube pattern (see pic on page 39) on top and attached on the bottom, like a hasselback. Heat the oven to 160C/fan 140C/gas 3.
Put 5 tbsp oil and butter in a large roasting tin and put in the oven for 3 minutes to melt and get hot. Add the potatoes, cut-side up, and spoon over the buttery oil. Roast for 1 hour, basting again halfway through.
After an hour, increase the heat to 220C/fan 200C/gas 7, baste again, and roast for another 20-30 minutes until golden and crunchy. Sprinkle with a good amount of sea salt and serve.