In a small bowl, mix together salt, pepper, seasoned salt, marjoram, mustard and cardamom. Rub spice mixture over lamb. Cut 16 deep slits into roast. Insert thyme and a sliver of orange peel into each slit. Place lamb fat side up on a rack in a shallow roasting pan. Insert meat thermometer in center of thickest portion of meat.
Roast lamb in preheated oven for 2 1/2 to 3 hours. Meat is medium done when a meat thermometer registers 175 degrees F. and is well done at 180 degrees F.
Remove meat thermometer. Place roast on a warm serving platter. Put a paper frill around end of leg bone and garnish platter with fresh mint.