Preheat oven to Gas 4, 180ºC, 350ºF.
Heat oil in a pan and add onion and garlic and cook for 2-3 minutes until softened, but not browned. Add chopped herbs and stir
Place boneless leg of lamb on a board and open out to flatten.
Spoon into the centre of the cavity the onion mixture and crumbled cheese. Spread out but not quite to the edges. Roll joint up and tie with heatproof string.
Weigh the joint and work out the cooking time:
Medium - 25 minutes per 450g/½kg (1lb) plus 25 minutes
Well done - 30 minutes per 450g/½kg (1lb) plus 30 minutes
Place in a roasting pan (any leftover stuffing place 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 new potatoes, beetroot wedges and rosemary sprigs and toss in the juices.
Serve joint thickly sliced with roasted potatoes and beetroot.