Fill the casserole dish with hay - well packed - and fill with water. Leave to soak for 20 minutes then drain.
Finely chop the garlic and half the rosemary leaves. Mix well with the butter to form a garlic and rosemary butter.
Dry the lamb and stab deeply all over with a sharp knife.
Line the casserole dish with a thick layer of damp hay, place the lamb on top and work the butter into its top and sides. Add the remaining rosemary sprigs around the lamb.
Cover the sides and top of the lamb with damp hay to fill the casserole dish - make sure the lamb is reasonably well centered. Put the lid on the casserole dish and place in the oven for 2 hours.
After 2 hours of baking at gas mark 5 turn the temperature up to gas mark 6 for 30 minutes.
Remove the casserole dish and its lid; allow the lamb to rest in the hay for 15 minutes.
Serve with roast vegetables and a thick gravy.... and don't forget the mint sauce!!!
