Combine first 6 ingredients in container of an electric blender; process 15 seconds or until smooth. Add oil to mustard mixture, one drop at a time; process on low speed until light and creamy.
Place lamb, fat side up, on rack in a shallow roasting pan; insert meat thermometer in lamb. Brush mustard mixture on lamb. Bake at 160ºC or 1 hour and 20-30 minutes or until meat themometer registers 160 degrees.