Combine first 6 ingredients in container of electric blender; blend 15 seconds or until smooth. Add oil, one drop at a time, to mustard mixture; blend on low speed until mixture is light and creamy. Place lamb, fat side up, on rack in a shallow roasting pan; insert meat thermometer in one of the racks, making sure it does not touch fat or bone. Brush mustard mixture on lamb. Bake at 350F until meat thermometer registers 175F to 180F (about 1 hour and 20 to 30 minutes). Serve over rice, if desired. Yield: 8 servings.