Peel the potatoes and cut them into long fries about 1/4-inch thick. As you cut them, put them in a bowl of cold water to cover.
Pour enough oil into a medium pot so that the oil comes 3 inches up the sides. Heat the oil to 135°C. Drain the fries, rinse them of excess starch under cold water, and pat dry. Working in batches if necessary, cook them in the oil for about 5 minutes; they should be soft and pale. Remove them from the oil using a slotted spoon and rinse under cold water to remove excess oil and starch for a crisper fry later.
Reserve the pot of oil. Cover a rimmed baking sheet with paper towels and lay the fries on top. Chill completely in the refrigerator. The potatoes can be prepared this way up to 1 day in advance of cooking them. Heat the oil to 180°C.
Add the fries, in batches if necessary, to the oil and cook, stirring gently, until the fries are golden brown, about 5 minutes. Remove them from the oil to a large paper-towel-lined bowl and season them with the rosemary and salt, shaking the bowl to distribute the seasonings evenly.