Take the leg of lamb out of the fridge 30 minutes before you want to cook it.
Heat the oven to 200ºC (400ºF).
Peel and slice the garlic cloves. Wash the rosemary, dry it, then scrape a knife over it to crush the leaves a little.
Put the garlic slices and rosemary in 1/4 of a cup of olive oil. Rub this mixture all over the lamb.
Wash and dry the cherry tomatoes.
Cover the bottom of the roasting pan with a thin layer of olive oil, then place the lamb leg in the pan. Arrange the cherry tomatoes around the lamb.
Roast for 30 minutes, then turn the oven down to 130ºC (260ºF). Baste the leg of lamb with the juices from the bottom of the pan every 20 minutes. Cook it for another 1 1/2 - 2 hours depending on how done you like your lamb.
Slice the lamb and serve it with the tomatoes and juice on top of it.