Preheat the oven to 180C. Wash and peel the potatoes and cut into quarters. Put them in a bowl under running water for five minutes to wash the starch off. Cover the potatoes with water in a large saucepan, bring to the boil and cook until very soft (about 25-30 minutes). Drain carefully and leave to cool in the colander. Pour five millimetres of olive oil into a roasting tray large enough to hold the potatoes in one layer. Place in the oven for 15 minutes. Add the potatoes to the tray with the garlic, if using. Stir to coat in the oil and place back in the oven for at least one hour, turning every 20 minutes. After one hour, add the rosemary and thyme to the pan, if using, and return to the oven for 15-20 minutes or until potatoes are golden brown and crispy all over. Drain on kitchen paper; season with salt