How to cook
Place the lamb in a large tray, at the bottom of which you've placed the coarsely chopped onions and unpeeled garlic cloves.
Pour in water and oil, cut the butter into pieces and distribute them inside the lamb and out.
Cover with foil and bake in a moderate oven for 8-9 hours, while basting the meat occasionally.
Remove the foil for the last hour of baking and leave the meat to crisp and gain a nice crust. If the water boils off, pour in more but make sure it's hot.