1) Preheat the oven to 180°C/gas mark 4. Pour 2.5cm of oil into a deep, heavy-based pot and heat it to 180°C.
2) Slice the potatoes into thin matchsticks (1/3cm thick) with a vegetable slicer or mandolin, dropping them into a bowl of cold water as you cut. Drain the potatoes in batches and dry them thoroughly with kitchen paper. Drop the potatoes in batches into the hot oil and cook for 3 to 5 minutes, until golden brown. Remove from the pot with a wire basket skimmer or slotted spoon and drain on kitchen paper. Place on a baking sheet, sprinkle with salt, and keep warm in the oven while you cook the rest of the potatoes.
3) Sprinkle the potatoes with parsley, if desired, and serve hot.