Cut the potatoes into a very fine julienne (very thin strips) in the food processor or use a good mandolin or grater. Thoroughly pat them dry. Add the rosemary and a little salt to the grated potatoes. Heat a thin coat of oil in a frying pan and fry 3 heaps of potato mixture in the oil at a time. Do not stir or touch! When the edges turn golden yellow, turn and fry on the other side. Allow to drain on kitchen paper. Can be easily made in advance: heat the biscuits for 6 minutes prior to serving in an oven preheated to 180°C (350°F/Gas 4).