1. Heat oven to gas 7, 220°C, fan 200°C fan. Pour the oil into a large roasting tray with the garlic and half the rosemary, add a big sprinkle of sea salt. Put into the oven to heat up.
2. Take each potato and cut into small chunks, keeping the skins on will add texture. Put into a pan, cover with water and turn up to the highest heat. Once boiling leave for 1 minute, turn off the heat, drain and leave to steam dry in a colander for a few minutes. Shake the colander to ruffle the edges a little.
3. Remove the oven tray carefully, add the potatoes, toss to coat in the oil, season and top with the rest of the rosemary. Roast for 30-35 minutes, until crispy and golden.