Preheat the oven to 160°C.
Use a sharp knife to make shallow slits in the lamb flesh (4-5). Mix together the butter, garlic and chopped thyme. Push butter mixture into the slits and sprinkle lamb with flaky salt and freshly ground black pepper.
Place the thyme sprigs and sliced onion in the base of a roasting tin and sit lamb on top. Drizzle over the olive oil. Place in the oven and roast for 2 3/4 hours.
Remove the lamb from the oven half an hour before the end of cooking time. Place lamb on a board, then tip all the juices into a saucepan. Replace lamb back into the roasting tin and return to the oven to finish roasting.
Remove from the oven and leave to rest, covered loosely with foil and a tea towel for at least 10 minutes before carving.
Skim the fat off the roasting juices in the saucepan. Add the beef stock to the remaining juices and bring to the boil.
Reduce heat and add the crème fraiche or sour cream, lemon zest, vinegar and bay leaf. Simmer until the sauce reduces a little ( it will be a thin sauce). Season with salt to counteract acidity. Strain sauce into a jug or gravy boat and serve hot with the lamb and vegetables.
Recipe from recipes.co.nz