Heat the butter and oil in a frying pan; add the onions and cook over a gentle heat until softened and just starting to colour.
Place the potatoes in the bottom of a roasting pan, spoon the cooked onions over them; season with salt and pepper.
Make small incisions all over the lamb and insert the garlic slices; place the lamb on a rack over the onions and potatoes. Season the leg well salt and pepper.
Pour the stock over the potatoes and roast in a hot oven (200c) for the first 20 minutes, then lower the temperature to moderated (180c) and cook a further hour. This will give you medium cooked lamb, if you like your lamb a little better done, cook for a further 40 minutes.
Carve the lamb in thick slices, lay it over the potatoes and onions, sprinkle with parsley and serve it in the dish it was cooked in Yum.