Preheat oven to 180ºC (355ºF). Place the stock, sherry, thyme and peppercorns in a baking dish. Wrap each piece of leek with a slice of prosciutto, place in the baking dish and top with the butter. Cover tightly with aluminium foil and bake for 40 minutes. Remove the foil and bake for a further 5 minutes or until the prosciutto is slightly crispy. Serves 4.