Preheat the oven to 200°C, fan 180°C, gas 6. Cut the peeled potatoes in half (or quarters if they are large), so that they are evenly sized. Put in a large pan and add enough water to just cover.
Bring to the boil, then cover and reduce to a simmer for 7-8 minutes. Drain the parboiled potatoes then return them to the dry pan. Give them a bit of a shake to rough up the edges.
Put the butter and oil in a large roasting tin and place in the preheated oven for 5 minutes, until very hot.
Remove the tin from the oven and carefully tip the potatoes into the hot oil and butter, along with the halves of garlic and the rosemary. Toss to coat and season with freshly ground black pepper. Roast for 40-50 minutes, turning once or twice, until golden and crisp. Serve immediately.