Preheat the oven to 190°C / 375°F / Gas mark 5.
Pierce the lamb all over with a sharp knife and push the garlic halves inside the holes.
Rub the oil all over the lamb and season well with salt and pepper.
Sprinkle half the thyme on top, than place the lamb on a wire rack over a roasting tray.
Roast for 30 minutes.
Meanwhile, in a small pan, bring the stock and butter just to the boil.
Carefully lift out the lamb and wire rack.
Pour the potatoes into the roasting tray and pour over the buttery stock.
Season with salt and pepper then add the remaining thyme.
Mix everything well, so the potatoes evenly distributed.
Place the rack and lamb back on top and cook for 1 hour.
Increase oven temp to 220°C / 425°F / Gas mark 7 after 1 hour and remove the lamb.
Cover loosely with foil and transfer to a warm place to rest for 30 minutes before carving.
Continue cooking the potatoes until they are golden and starting to crisp.
Cook the peas according to the packet instructions, drain and cover until ready to serve.
Return the lamb to the top of the potatoes, pour over any resting juices over everything and serve with the peas.