Scrub and peel the potatoes. If your potatoes are around the size of a large egg, keep them whole. Otherwise, halve or quarter them so they are all roughly the same size.
Place 1 potato in a serving spoon or a wooden spoon – this helps to keep it steady. If it is halved or quartered, then place it flat side down. Slice the potato at 3mm intervals, but don’t slice all the way down. Make sure the bottom 1cm of each potato is unsliced. Once they’re sliced, pop the potatoes into a pan or bowl of cold water and cover. Set aside till you’re ready to cook.
When you’re ready to roast the spuds, make sure your oven is heated to 200°C/Fan 180°C/Gas 6.
Place a large roasting tin over a couple of hobs. Add the butter with 3 tbsp oil. Season with a good pinch of salt and pepper. Warm over a medium heat to melt the butter, then add the potatoes and turn them once or twice, spooning the butter and oil all over the potatoes so they are well coated.
Slide the tin into the oven. Roast for 50-60 mins till they are crisp and golden all over, turning them half way through. To check they’re cooked, push a skewer into one of the larger spuds – it should go through easily. Transfer the hasselback roasties to a warm serving dish and serve.