Wash and brush the potatoes thoroughly. Cut the potatoes across at approx. 3mm intervals, heat the butter and oil in a baking dish, add the potatoes, turn them well and use a spoon to drip fat into the incisions. Place the potatoes in the pan with the incisions facing up, season with salt and pepper and bake for approx. 45 minutes to 1 1/4 hours (depending on size) on the middle rack until they are soft.