Cut 1-inch-deep slits, 2 to 3 inches apart, into leg of lamb. Insert a garlic slice and a small piece of rosemary into each slit. Place lamb on a rack in a roasting pan.
Bake at 450° for 5 minutes. Decrease oven temperature to 350°, and bake 1 hour and 30 minutes or until a meat thermometer inserted into thickest portion registers 145° (medium rare). Remove lamb from oven, and let stand 15 minutes.