Put the lamb in a large roasting tin. Using a sharp knife, make about 12-15 incisions all over the flesh.
To make a marinade, combine the garlic, chopped rosemary, lemon juice, olive oil and vinegar in a bowl. Season the leg, then rub the marinade over the skin, pushing it deep into the cuts. Leave to marinate for 1 hour, or chill overnight (allow to come up to room temperature before roasting).
Preheat the oven to Gas Mark 7, 220°C, fan 200°C. Roast the lamb for 20 minutes, then remove from the oven and stuff the rosemary sprigs and lemon rind into the incisions.
Reduce the temperature to Gas Mark 4, 180°C, fan 160°C. Scatter over the cumin seeds and return the lamb to the oven for 1 hour 40 minutes for medium meat or 1 hour 50 minutes for well done.
Meanwhile, combine the minty yogurt ingredients in a bowl with some seasoning and a squeeze of lemon juice to taste, and chill until needed.
Remove the lamb from the oven, cover and rest for 30 minutes. Serve on a bed of watercess, with griddled lemon wedges and the minty yogurt.
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home and view the full Food safety at home video.
Why not pair zesty lemon and warming cumin in this roast lamb centrepiece with this full-bodied, aromatic finest* Vina Mara Rioja Reserva red wine.