Preheat oven to 180ºC/350ºF/ Gas Mark 4. Heat the oil in a pan and add the onion and garlic and cook for 2-3 minutes until softened, but not browned. Add the chopped herbs and stir.
Place the boneless leg of lamb on a board and open out to flatten. Spoon the onion mixture and crumbled cheese into the centre of the cavity. Spread it out but not quite to the edges. Roll the joint up and tie with heatproof string.
Weigh the joint and work out the cooking time. For medium, allow 25 minutes per 450g plus 25 minutes. For well done, allow 30 minutes per 450g plus 30 minutes.
Place in a roasting pan (place any leftover stuffing on the bottom of the pan under the joint), season and drizzle with oil. Roast, basting occasionally.
40-50 minutes before the end of cooking time add the new potatoes, beetroot wedges and rosemary sprigs and toss in the juices. Serve the joint thickly sliced with roasted potatoes and beetroot.