Preset oven Gas Mark 4, 180°C (350°F).
Put the garlic, ginger, lemon rind, parsley, mint and rosemary in a food processor. Switch on for a minute or two until everything is finely chopped. Switch on again and add the oil slowly until you have a bright green mixture. Set the mixture aside.
Place the joint in a roasting pan with the onion and water, cook, allowing 20-25 minutes per ½ kg (1 lb). Thirty minutes before the end of cooking time remove from the oven, spread the herby mixture over the joint. Return to the oven and finish cooking. Keep an eye on the water level. Top it up if necessary.
When the lamb is cooked remove from the pan and keep warm. Discard the onion and remove excess fat. 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.