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 degree F oven for 1 1/2 to 1 3/4 hours or until the thermometer registers 150 degrees F (medium).
For sauce, reserve pan juices; skim fat. Add water, if neccesary, to measure 2/3rds cup liquid total. In a medium saucepan stir together yogurt, cornstarch and bouillion granules. Stir in juice mixture. Cook and stir until thickened and bubbly. Cook and serve with sliced meat.
