Heat the oven to Gas Mark 7, 220°C (450°F).
To cook the lamb: Place the lamb skin side up in a roasting tin. Put the ingredients for the herb crust in a small bowl and mix well. Then spoon the mixture over the lamb. Place in the oven and roast for 40 minutes.
To make the Salsa Verde: Put all the ingredients, except the oil, into a blender. Turn on and gradually add the oil.
When the lamb is cooked, transfer it to a carving board, then leave to rest in a warm place for 10 minutes.