Trim fat from lamb; cut four evenly spaced 1" deep slits in the surface.
Combine lemon peel, juice, rosemary, thyme and pepper; rub onto meat and into slits. Place meat on the rack of a roasting pan.
Insert a meat thermometer into the thickest portion. Roast in 325°F oven 1 1/2 to 1 3/4 hours or until the thermometer registers 150°F.
Reserve pan juices; skim fat. Add water, if necessary to measure 2/3 cup of liquid total.
In a medium saucepan stir together yogurt, cornstarch and bouillon granules. Stir in juice mixture. Cook and stir until thickened. Serve with sliced meat.