Trim fat from lamb. Make several small slits in lamb; stuff with garlic slices. Combine yogurt and next 5 ingredients (yogurt through bay leaves) in a large zip-top bag; add lamb. Seal and marinate in refrigerator for at least 8 hours.
Preheat oven to 450°.
Remove lamb from bag, reserving marinade. Place lamb on a broiler pan; insert meat thermometer into thickest portion of lamb. Pour marinade over lamb; arrange potatoes around lamb. Lightly coat potatoes with cooking spray, and sprinkle with salt. Bake at 450° for 15 minutes. Reduce oven temperature to 325° (do not remove lamb from oven). Bake an additional 1 hour and 15 minutes or until thermometer registers 145° (medium-rare) to 160° (medium). Let stand 10 minutes.
