Puncture the leg of lamb with the tip of a knife just far enough to insert slivers of garlic into the holes. Mix together the dill, salt and rosemary and rub over the leg of lamb. Place the lamb, fatty side up on a rack in a shallow roasting pan.
Roast uncovered for 2 to 2 1/2 hours in the preheated oven for medium. Cover with aluminium foil and let stand for 15 to 20 minutes before carving. The lamb will continue to cook a little bit and the juices will set up better for carving.