Trim the lamb of sinew and excess fat, keeping it in one piece. Roll up and tie with kitchen string. Place a large, wide heavy-based pan over a high heat and add the vegetable oil. Once hot, quickly sear the lamb on all sides, colouring it evenly. Remove to a plate.
Add the onion, carrot, leek, thyme, rosemary and garlic to the pan and cook for 3-4 minutes to s