Greeks prefer their lamb cooked through--as in well done. It's not a dry, unpleasant well done, mind you, but from a long, low and slow dry heat that results in a significant (often deeply seasoned and lemony) exterior crust, and an almost fork-tender interior. Things will shrink, but flavors will also concentrate. Turn the lamb fairly frequently. Meat juices will stay in motion, and you'll get an evenly moist interior.