Heat the olive oil in a saucepan, add the onion and all the ingredients except the couscous and parsley, season with salt and pepper and cook stirring for 5 minutes, adding a little water if it starts to stick. Remove from the heat and stir in the couscous and parsley.
Pre-heat the oven to 160°C.
Lay the lamb onto a flat surface and open up (butterfly) gently with a knife. Spread the butter paste onto both sides of the lamb. Spread thecous cous filling onto the lamb and roll it up tightly, sercure with string or tooth picks.
Layer the base ingredients on the base of a casserole or deep oven tray. Place the stuffed lamb on top and pour over the chicken stock. Cover with foil and roast for 2 hours or until cooked.