Set the oven to 250 degrees C, air circulation, rack in the middle.
Wash and peel the potatoes, then cut them in long sticks of 1 cm each side. Put a large pot filled with water on high fire and bring to a boil. Add the cut potatoes, bring back to boil and keep for 3-4 minutes. Drain them and let cool.
In a large bowl add the potatoes and drizzle with the oil. Toss to coat well.
Use an oven-proof baking dish and spread in the potato sticks. Add the rosemary sprigs and season with the salt and pepper.
Bake for 20-25 minutes until golden brown making sure that halfway through the cooking you toss and turn them.