Preheat the oven to 220°C/200°C fan/gas 7.
Rub the lamb all over with olive oil and season generously with salt and pepper.
Place the lamb in a roasting tin and scatter the rosemary and garlic around it.
Roast for 20 minutes, then reduce the temperature to 180°C/160°C fan/gas 4 and continue to roast for 1 hour and 10 minutes for medium-rare, or until cooked to your liking.
Allow to rest for 15 minutes before carving and serving.
