Preheat the oven to 230°C.
Make slits 4 cm apart in the lamb and insert garlic slivers and rosemary sprigs.
Rub the lamb with softened butter and olive oil in a roasting tin.
Roast the lamb for 30 minutes.
Peel and finely slice the potatoes and onions then season in a bowl.
Remove the lamb from the tin and layer the potatoes and onions into the meat fat.
Pour 455 ml of stock over the vegetables.
Place the tin on a low shelf and the lamb directly on the oven rack above it.
Roast for another hour until the meat is cooked to your preference.