Leg of Lamb with Oregano & Lemon

This is possibly one of my favourite meals. It is soft, wonderful and meltingly lemony and we would sometimes eat the leftovers for breakfast before school. I’m not sure if this is totally the Greek way of making it — maybe the butter and cooking time are my mother’s addition — but this is how it is. My mother says you could cook it overnight: I never have, but I believe her.