Preheat oven to 450° F.
Trim fat from lamb; cut four evenly spaced 1-inch deep slits.
In a mixing bowl combine lemon juice, grated lemon peel, rosemary, thyme, and pepper. Rub onto the meat and into the slits. Place roast on rack in roasting pan; sear meat in oven for about 15 minutes or until browned throughout.
Reduce oven temperature to 250° F. to 300° F.
Insert a meat thermometer into the thickest portion. Roast for 1 1/2 to 2 hours or until the thermometer registers 130° F. to 140° F. for medium-rare or 150° F. for medium.
Remove from oven. Cover with foil and allow to stand for about 20 minutes before slicing.
For the sauce: Drain pan juices into a measuring cup and skim off the fat. Add water, if necessary, to measure to 2/3 cup. In a saucepan, stir together yogurt, cornstarch, and beef bouillon. Stir in pan juices and cook, stirring constantly, until thick and bubbly. Serve with the sliced lamb.
Servings: 6