In a small bowl mix together salt, pepper, seasoning 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.
Roast lamb in preheated oven for 2 1/2 to 3 hours. Place roast on a warm serving platter. Garnish with fresh mint.