1. Preheat the oven to 180°C/160ºC fan/gas mark 4.
2. Wash and peel the potatoes and cut into quarters. Put them in a bowl under running water for 5 minutes to wash the starch off.
3. Cover the potatoes with water in a large saucepan then add the garlic and rosemary, if using, and bring to the boil.
4. Cook for about 25-30 minutes or until the potatoes are very soft and almost breaking apart. Drain carefully and leave to cool in the colander.
5. Meanwhile, pour 5 mm of olive oil into a roasting tray large enough to hold the potatoes in one layer. Place the tray in the oven for 15 minutes to preheat the oil.
6. Add the potatoes to the pan and coat them in the hot oil. Place the tray back in the oven for at least 1 hour and 15 minutes, turning the potatoes gently every 20 minutes, until the potatoes are golden brown and crispy all over.
7. Drain on kitchen paper and season with salt.