Heat oven to 190C/170C fan/gas 5.
Pierce the lamb all over with a sharp knife
and push garlic halves into the holes. Rub
the oil all over the lamb and season well.
Scatter half the thyme on top, than place
the lamb on a wire rack over a roasting
tray. Roast for 30 mins.
Meanwhile, in a small pan, bring the
stock and butter just to the boil. Carefully
lift out the lamb and wire rack. Tip the
potatoes into the tray and pour over
the buttery stock. Season and sprinkle
on the remaining thyme. Mix everything
well, shaking the tray until the potatoes
are in an even layer. Place the rack and
lamb back on top and cook for 1 hr.
After 1 hr, increase oven temp to
220C/200C fan/gas 8 and remove the
lamb. Cover loosely and transfer to
a warm place to rest for 30 mins before
carving. Continue cooking the potatoes
while the lamb rests until they are golden
and starting to crisp. Return the lamb to
the top of the potatoes, pour over any
resting juices and carve over the
potatoes so all the juices fall onto them.