Preheat the oven to 220C or 200C Fan or 425F or Gas 7. Place the preferred fat into a roasting tray and the roasting tray into the centre of a preheated oven.
Bring a large pan of salted water to a boil and cook the prepared potatoes for 15 minutes until they are just cooked. Allow the potatoes to dry thoroughly in the colander for a few minutes.
Add the potatoes, garlic, and rosemary to the hot fat. Season the potatoes to taste and baste them in the hot fat.
Roast for 45-60 minutes. Turn the potatoes over at least once during cooking for a more even colour and crispiness.
Drain the fat off and serve immediately.