Preheat the oven to 220°C (430°F) Gas Mark 7.
Place the lamb in a roasting tin. With the point of a knife make deep holes all over the side of the lamb facing upwards. Divide the rosemary into little tufts and push these into the holes. Drizzle some olive oil over the lamb and season with salt and pepper. Add the chopped onion and water to the roasting tin.
Put in the oven and roast for 20 minutes, then reduce the heat to 180°C (350°F) Gas Mark 4 and continue to roast for another 50 minutes. The lamb will be cooked medium at this stage. Keep an eye on the water level. Top it up if necessary.
Meanwhile cut the bulbs of garlic in half across the middle and place them in another roasting tin. Peel and quarter the potatoes, carrots and red onions and add them to the garlic. Mix with a little olive oil, bay leaves and seasoning. Roast for the last 50 minutes alongside the lamb.
When the lamb is cooked remove it from the oven. Wrap it loosely in foil and allow to rest for 15 minutes before carving. Discard the onion from the roasting tin. Then add a dash of wine to the pan juices. Boil up and reduce to a nice syrup. This will take 2-3 minutes. Season and serve with the sliced lamb and roasted vegetables.