Greek leg of lamb

Butterfly the lamb as follows: Keeping the blade of your knife horizontal, partially slit open the cavity left by the leg bone. Turn the flap outwards, like the page of a book, and spread the meat flat. Working from the centre, make a similar horizontal cut in the thick muscle opposite, so that it can be opened out flat.
Mix olive oil, marinade and garlic and marinate the meat overnight. Brown the meat quickly in a little heated oil to seal it.
Oven roast for about 1 1/4 hours at 160 ºC until done but still pink inside. Serve with yoghurt sauce which is simply made by mixing together 100 ml chicken stock;
250 ml natural yoghurt; 10 ml cornflour
20 ml freshly chopped parsley and 25 ml lemon juice and heating it all together for two minutes in a pan.