Score the fatty top side of the lamb about 1/4 inch deep. Season the lamb all over with salt and pepper. On a large rimmed baking sheet, toss the whole garlic cloves, onion, carrots and celery with a generous drizzle of olive oil. Arrange the vegetables in an even layer and season with salt and pepper. Scatter the rosemary sprigs over the vegetables and set the leg of lamb on top, fatty side up. Spread half of the garlic-and-herb rub all over the lamb, making sure to rub it into the score marks. Roast the leg of lamb for 20 minutes.