Preheat the oven to moderate 180C
(350F/Gas 4). Cut small deep slits in the lamb and push a piece of
garlic into each slit. Combine the oil, rosemary, salt and pepper, and
rub the mixture over the lamb using your fingertips. Arrange the tomato
halves in the base of a baking dish. Top with the potato slices and
place the lamb gently on top. Roast for 1 hour, turning the lamb once.
Reduce the oven temperature to warm 160C (315F/Gas 2-3), and bake for 10
minutes for medium, or 25 minutes for well done.
Remove the lamb, wrap tightly in aluminum
foil and leave for 15 minutes before carving. The tomatoes and potatoes
can be returned to the oven and cooked for a few more minutes, while the
meat is standing, to crisp the potatoes. (If the tomatoes are sticking
to the baking dish, sprinkle them with a little water to moisten.)