Using a large rimmed baking sheet, scatter the whole garlic cloves, onions, potatoes, carrots and celery, drizzle them with olive oil. Season with salt and pepper. Scatter the sprigs of rosemary over the vegetables and set the leg of lamb on top., fatty side up. Spread 1/2 of the garlic and herb mixture all over the lamb, be sure to rub it into the score lines. Roast for 20 minutes.