• 1 kg (2 lb 4 oz) floury potatoes, such as king edward, peeled and quartered
• 2 tablespoons extra virgin olive oil
• 40 g (1½ oz) butter
• 1 teaspoon rosemary leaves

Preheat the oven to 200°C (400°F). Put the potatoes in a large saucepan and cover with cold water. Bring to the boil and cook for 5 minutes. Drain in a colander and leave to steam-dry for a few minutes. Toss the potatoes around in the colander to roughen up the surface.

Put the olive oil, butter and rosemary in a large roasting tin and sprinkle with a generous amount of sea salt and freshly ground black pepper. Put the tin in the hot oven for 5 minutes to heat.

Add the potatoes to the hot tin and roast for 1 hour, turning the potatoes every 20 minutes, until golden and crispy.